No. copy/paste from gmail or any email works fine like this. I have used it countless times.
long press on the email...anywhere. it will give you the option to "copy text"....select that. THEN run your finger tip over the text that you want to copy. It will highlight it and when you let go it will copy the highlighted part to the clipboard. then paste it where you want.
Do you have an Eris? If you open up an email message in gmail, and long press - nothing happens. There are three buttons on the bottom - "Archive", "Labels" and "Delete". Press "Menus" and there are more options - "Archive", "Add Star", "Mark Unread", "Change Labels", "Delete" or "More", which yields "Back to Inbox", Mute", Report Spam". Nowhere is there an option to copy text.
If you long press on a message in the message list in the in box, you get a menu with "Read", "Archive", "Mute", "Mark unread", "Delete", "Add star", "Change labels", "Report spam".
You can copy text with a long press from the "Mail" app, however. You simply cannot from the gmail app on the Eris. I prefer the gmail app - it pushes mails, it has the ability to archive mail in gmail, move to a different label, etc. The Mail app does allow you to move messages to a different label, though not as easily, and it is not push mail - it must be set up to poll for messages at a specific time interval. I prefer push myself.