There is a massive leap from U23 level, or pre-season friendlies, to the Premier League in terms of the speed and physicality of the game. This gulf is much wider than in the past, as it used to be the case that more first team players would regularly play in the U23s to maintain sharpness so the quality of the U23 matches was more competitive and players coming up would get a better taste of what it was like to play against grown men. Now most clubs are trying to use that level to produce young talents so its mainly a group of 18-20 year olds playing against another group of 18-20 year olds, none of whom really have any idea what its like to play at a very high tempo, with a couple older players who are clearly never going to make it thrown into the mix and maybe a senior player coming back from an injury. This is why most clubs are going the loan route with their best young players - the step up from U23 to Premier League is just too big and reserve competition just doesn't prepare players for the rigors of the Premiership.
With a guy like Gomes, these issues are probably even more significant because of his size. Players that small can be successful, of course, but they usually need some particular qualities - most importantly really good acceleration and change of direction (which Gomes may have), a stocky build and a very strong lower half helps as well (not seeing that in him) - and they need to learn how to use those qualities against grown men flying around the pitch at speeds they've never faced and physically manhandling them at every opportunity. Moreso than other types of players his age with different physical profiles, Gomes is unlikely to be ready for the EPL and would probably benefit a lot from a loan spell.