I think I misunderstood what you were asking. I thought you just couldn't see your Gmail contacts at all but they are synchronized to your phone's list of contacts and you might not have been seeing them if your contact listing was set to Phone Contacts or Facebook Contacts, etc. Setting the display group to Google Gmail will display them. That's not what you were asking though!
Now that I know you are trying to display a list of contacts when you are composing a message, I now see you are right. There apparently is no way to see a list of contacts when you select the To: field. That is a bit lame indeed. As Technoman has pointed out, you could just start typing and it will automatically select from your list of contacts. When it autocompletes enough of the right contact, you are on your way.
HOWEVER, I find that typing the contact is not the easiest way for me. Instead, I select the contact differently. I open Contacts, quickly scroll to the person I want to email, select them and the contact opens, I select the @ icon next to their e-mail address, and it pops up a box asking if I want to use the built in mail or Gmail and I select Gmail and POOF! I'm in the Gmail compose screen in the Subject field ready to go. No typing of someone's name or address at all. It's very fast.
THAT to me is the easiest way.