One of the most commonly reported issues with Gmail in Opera is that the left column is rendered too wide, forcing you to scroll left and right to read your mail. Here's a screenshot of what the problem looks like:
Here's how to fix it:
Step 1: Login to your Gmail account in Opera.
Step 2: With the Gmail tab open, press the [F12] key (Windows/UNIX) or [Option]+[F12] (Mac OS X). This opens the Quick Preferences menu. Choose "Edit Site Preferences" in this menu.
Step 3: Click on the "Network" tab. Under "Browser identification", choose "Identify as Opera".
Step 4: Click on "OK" and then press [Ctrl]+[R] (Windows/UNIX) or [Cmd]+[R] (Mac OS X) to reload Gmail. (If that doesn't work, try restarting Opera)
Hopefully this makes it possible for you to use Gmail in Opera without having to spend all your time scrolling back and forth . If you still see the issue where Gmail's left column is too wide, please post a comment and include your "Browser:" information from www.ipchicken.com, and I'll see what I can figure out!