Gomez should be tested as central defense with Matip. He can't do worse than Klavan or Lovren.
Gomez is very defensive and can play comfortably in that position.
Where would you play him then? Move Matip to the left and have Gomez play next to Trent? I don't think it's a good idea to have two, while both very talented, inexperienced u21 lads playing next to each other in PL or CL games. They would constantly get targetted, just like Trent was in the City game.
Or have him play next to Moreno / Robertson? Both aren't optimal circumstances to start a young CB in. I'd rather ease him in next season, on the back of a season with plenty of games as a RB, than throw him out there against PL or CL opposition.
Clyne is also out for a few months, playing Gomez means playing Trent too. If one of them gets injured, we're left with one RB having to play all the games. Can't take that risk.