If Walker's injured then this is an interesting early test of how Pep sees Gomez's role.
When this happened last season, he chose to play Stones at right back rather than move Cancelo to the right and play Zinchenko. Admittedly there may have been some non-footballing reasons why he didn't want to play Zinchenko, especially at first, but this happened multiple times. I thought at one point that it was because Pep so strongly preferred Cancelo on the left to the right - but in that case not buying a right back this summer doesn't make sense.
I'd like to see Gomez start. If it doesn't work out, we can always bring on a other centre back and either play Ake on the left or move Cancelo left and play Stones on the right. But we've bought a full back, he looks good from what we've seen, and this is a perfect opportunity to test him in Europe.