What iGPU is the real question.
The Steam Deck has 4C/8T Zen 2 and 8 CUs. If this is the same (and it would require LPDDR5 support), then it will be a decent basic gaming CPU.
If on the other hand we're looking at 6 CUs and down (or LPDDR4) then we're probably back to Vega levels, which isn't bad but not as exciting.
Not that it's strictly bad. 4C/8T Zen 2 + 4 CUs will likely have about the performance of a 5300U, and that's still usable for gaming.