Well it is probably set up to be automatic, but to check go into:
menu, settings, accounts & sync. Select your google account, and make sure the box next to sync contacts is selected. To futher ensure they are synced hit "sync now" down at the bottom. That should do it. If you want further verifcation you can log into your gmail account on the computer and select "contacts." You should be able to see all of the contacts you have on your phone there on your computer as well.