Upload videos from iPhone to YouTube
What I would do is go directly to the Youtube app (I am testing this with my old 3GS so this should work) and drag to the left to access all of the different menus, then click on the little up arrow next to my channel (assuming you are already logged in on the app) and then you just click on the video you want to upload instead of using the photos app and 'sharing' to Youtube. You could also try logging in to Youtube on a computer as on there it should give you the option to create a new Youtube account with your Gmail assuming that the cause of the problem is that your gmail is not tied to any Youtube. Assuming it let you create a new Youtube associated with your Gmail you should just be able to go back to the app on your iPhone and try again.