My cousin just got an HD2 & he's having problems syncing his contacts from his Gmail account onto the device (coming from a G1).
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Here are some instructions if my memory serves me correct.
From the home screen slide the slider over to mail and hit new/add account. You want to add an exchange/outlook account.
On the first page enter your gmail address and password then hit manual setup button underneath.
For username enter you entire gmail address again and enter your password again if it is not already there.
If you entered your entire email address ***@gmail.com then the domain field should be greyed out. Even if it is not greyed out just skip it since you domain is gmail.com it is taken care of.
Check the box that says save password.
For the server enter: m.google.com then check the ssl connection checkbox.
Hit next make sure you only have mail/contact/calender set to sync. If you have tasks set to sync it will give you an error since gmail doesn't support exchange tasks sync.
I hope that all makes since. It is an incredible pain to delete an exchange account so i cant go through it from the beginning but i think that is correct.