Sadly, the answer is 'not really all that much' at least as far as the ancient 'grom' of Outland are concerned. We know they were massive, vaguely humanoid beings who died at some point in the past and whose bones served Gul'dan in his creation of Temples of the Damned. When they died and how they're related to the other intelligent races of Draenor we have no idea (well, presumably they're native to Draenor and not immigrants to it like the Draenei were). We don't even know if they were related in any way to the gronn. Certainly the similarity between the words 'grom' and 'gronn' implies some kinship. Perhaps the orcs borrowed the word from the ogres. However, the bones used in the construction of Gul'dan's Temples have two eyes. The gronn we know of today only have one.
The gronn themselves (at least the ones we know of in Outland) all derive from Gruul the Dragonkiller. It's not known at this time if Gruul is the father of all gronn in existence or just an extended clan of gronn in the remnants of Draenor we still know about. Gruul has seven sons, Goc, Maggoc, Slaag, Durn the Hungerer, Gorgrom the Dragon-Eater, Grulloc, and Skulloc Soulgrinder. As of this writing there has been no mention of a wife or daughters of Gruul. Indeed, I have no idea how gronn reproduce and frankly I may be better off that way. In addition to these seven sons, there are lesser gronn known as mountain gronn and a named gronn called Thuk the Defiant who is either not related to Gruul at all or is at least not one of his sons. Gronn are often described as the 'immortal demigods' of the ogre race, and while that might be hyperbole (clearly gronn can die by violence) it's not by much.
The gronn are notable to us (especially as we approach the Cataclysm) as being one of the few forces on Draenor or Azeroth who have ever had much success combating the Black Dragonflight. Indeed, black dragons hate gronn as they do few others, because few others can claim to have actually hunted and killed their kind with such success. Baron Sablemane, a black dragon agent in Blade's Edge, makes it clear that he hates the gronn. "My loathing of interruptions is overshadowed only by my hatred of Gruul the Dragonkiller and his seven sons!" Gruul himself actually dared, alongside Khadgar, Alleria and Turalyon of the Alliance Expedition, to attack Deathwing himself directly. However, as mighty as Gruul is, he was no match for the father of all black dragons and would have most likely died if not for Khadgar's magical attack on Deathwing's adamantite armor plating.
Whether there are other gronn than those related to/descended from Gruul is unknown. (Tantalizing hints about Nath, the supposed ogre war god and namesake of the Mok'Nathal, hint that he might be a rival gronn, or perhaps something else entirely... one of the grom?) We do, however, know more about their supposed descendants, the ogres of Draenor.
The ogres, despite being descended from the gronn, suffered horribly at their hands and it seems likely that the tendency of ogres to settle anywhere they can comes at least in part from a desire to get as far away from them as possible. The ogres of the Blade's Edge Mountains are all firmly under the domination of Gruul and his sons (at least until a band of adventurers kills them all in assistance to the Ogri'la faction of ogres, ogres that have managed to increase their native intelligence through exposure to mysterious Apexis Crystals) who rule via their domination of High King Maulgar.
One of the interesting things about ogres is demonstrated by Maulgar: as a race, they're extremely malleable and prone to interesting variations. Maulgar, for instance, is an intermediate type known as an ogre lord with characteristics which make him almost seem a gronn/ogre missing link. While this places him firmly in the 'inferior to the gronn' camp (for all their brutishness the gronn see themselves as immortal, nearly godlike) his raw power was sufficient to place him firmly in command of the various ogre clans in Blade's Edge.
Still, when Gul'dan rose to power in the Horde quite a few ogre clans preferred him to the gronn and defected. For one thing, under Blackhand and Gul'dan the ogre race and its raw power was valued for use as shock troops against the draenei. Every ogre that died against their enemies was five or more orcs who didn't have to. Secondly, while the gronn used the ogres casually and even abused them on a whim, Gul'dan treated them as valuable allies. Sure, being Gul'dan he was fully willing to sacrifice them if it suited his needs, but he didn't waste them needlessly. Not all ogres joined the Horde, but even many that didn't took advantage of the Dark Portal to flee to Azeroth in an attempt to escape their gronn overlords.
Eventually, Gul'dan went so far as to experiment with ways to improve his ogre servants. This led to the carving up of elven runestones into the now famous Temple of Storms. (Gul'dan had a thing for making temples.) The power of the runestone lead to the creation of the ogre magi, ogres with increased intelligence and the ability to cast spells. (Cho'gall
is one example of this kind of ogre.) Ogre magi have since become reasonably common in ogre society, often serving as leaders or right hands to leaders of ogre tribes. Maulgar, for instance, seems to have been assisted by four ogre magi. While originally these ogres were the servants of Gul'dan and later Ner'zhul as leaders of the Horde, a great many of the ogre magi currently in existence owe no allegiance to such masters... Mog'dorg the Wizened
, for example, appears to be an ogre magi and is a member of the Ogri'la faction of 'evolved ogres' who seek enlightenment and freedom for all ogres from the hands of the gronn.
Following the old Horde's defeat, both those ogre clans that were allied to them and ones that had never been their allies scattered throughout Azeroth. You can today find ogres both in Kalimdor and the Eastern Kingdoms. (There are few
if any ogres i
n Northrend, possibly due to the Vrykul presence in the niche of 'gigantic humanoid' that ogres tend to try and fill wherever they go.) In addition to the ogre, ogre lord and ogre magi variants (and some ogres seem to be born with a second head, this appears to be more common in ogre magi but not limited to them) we have the Mok'Nathal to consider.
The 'Sons of Nath' are apparently crossbred from ogres and orcs. While Rexxar is often described as being half ogre, this may not be directly true, as his father Leoroxx
appears to be a massive brown orc similar to one of the Mag'har (but far, far larger) and he states "Know this: being born a half-breed does not make you Mok'Nathal
." (Note, it's possible that Leoroxx is himself a half-ogre and therefore Rexxar is one by virtue of being the offspring of one or perhaps two, as little is known of Rexxar's mother or her lineage.) Since Leoroxx's model is not the unique model Rexxar uses in Burning Crusade
and Rexxar himself used the model of a large orc until BC
released we get few clues from the game itself. It seems safe to assume that some if not all of the Mok'Nathal are half ogres, but that being a half ogre is not enough to be one of the Mok'Nathar, implying that they are a clan to themselves. Being that they take their name from an ogrish war god, it seems likely that there would have to be some connection to be explored.
Perhaps, if we ever return to find the remainder of Draenor, we'll learn more of the grom/gronn/ogre connection. At present, the ogres are the least and most common remnants of the giants of Draenor.