There is a slight lack of anywhere to put down tube shifters on your GT Traffic frame. as above, it's not worth it, too may parts to replace and fitting issues.

Also if you really want friction, you need to be looking at an old bike (15 years +), as most if not all road bikes made now are indexed with STI's (brifters) except the lowest end of the market ones. Would suggest you have a go with a bike with STI's, as correctly set up, they make riding a lot easier than with friction shifters