Here is how to export your Gmail contacts to your iPad Contacts list, so they will autofill when using the Mail app.
- Navigate to your contacts page and choose "Export…" under the More menu.
- Choose All Contacts, and use the vCard format.
- Open the downloaded file.
- When Address Book asks if you're sure you want to import them, choose yes, and quit Address Book when it's done.
- Select your iPad in iTunes and navigate to the Info tab.
- Turn on Sync Address Book Contacts if it isn't already, and customize the settings how you like.
- Sync your iPad with iTunes.
Done! I did notice from your question, though, that you're using the Gmail app. This fix will only work if you use Mail.app; for more information regarding setting that up, see this Apple help article.
Also: if you're running Windows, replace steps 3 and 4 with the instructions on this page.