Simple fact of the matter is he didn't warrant a place in the squad after last season and those two you mentioned despite getting virtually no minutes themselves deserved the call up ahead of him after both having very impressive seasons.
If I was a Newcastle fan, I'd be gutted Gordon got picked and gave up a months rest for 4 minutes of football and will now miss pretty much all of pre-season, likewise for Bowen if i was a hammer.
Grealish got substantial genuine rest and a kick up the arse he needed. He's had everything he needs over the summer to up his game and prove himself from day 1 of the season now and along with the others who reported back today, has a major advantage over the rest of the squad.