I've tried to use these desktop-inspired features on iPad and they are befuddling. Using two apps at once is beyond confusing. A simple use case, like copying an address from a Safari tab into Gmail via split screen requires so many mysterious, undiscoverable gestures, I doubt almost anyone is doing that. I think Jobs was right that convergence between workstations and mobile devices requires too many sacrifices, at…
I don't think this is as much of a problem as people make it out to be. They've managed to keep the iPad just as accessible to everyone, and they prod you occasionally with notifications from the Tips app for people who are willing to learn more.
As far as moving a URL from a Safari tab to Gmail, why not just copy and paste it? Tap once on the URL bar, the keyboard pops up with all the text selected, and there's a copy button right there. Or you can tap on the selected text to bring up the cut/copy/paste buttons.
Pressing and holding the URL to initiate a drag and then dropping in a text field also works for me, but I didn't test in Gmail. It's possible Google has some custom behavior that breaks this.