Well, I certainly hope that Pep knows what he's doing (in general, there's plenty of evidence over the last fifteen years or so that he does!). I don't frankly know anything about Gomez, but he is young, so that's in his favour. He's played at every youth level for Spain, not that that proves much (Patrick Roberts played at every youth level for England). I see that he's actually shorter by four centimetres than Zin, who's already no giant, so don't rely on him to win those headers against burly bustling centre forwards.
I can sort of understand that Pep and Txiki didn't really want to go and spunk £30 to 35 million on any of the other candidates. The only one he genuinely fancied was Cucurella, and while Cucu is obviously a useful full back, his price was downright ridiculous. And if you're spending £30 million on a player that you don't really want, that looks very expensive too.
We'll see. Hey, perhaps we'll have so much possession that it won't really matter. Cancelo, after all, doesn't really play left back at all. He's got licence to be all over the place.