I agree with you, we shouldn't blame Arsenal fans for expecting many goals from Gyøkeres next season since the hype on the player was too strong. The player rejected several clubs and is only interested in playing for Arsenal. Gyøkeres must perform at the high level expected by fans and the team for the next season. The fans hope to see the Gyøkeres who scored many goals at Sporting CP last season, so he should not disappoint.
However, Gyøkeres has previously played for Brighton and made a few senior appearances before being loaned to another club, should get used to the Premier League environment easily. Despite the fact I am not an Arsenal supporter, I have high expectations for the team's success next season because they are gathering key players.

I think most Arsenal fans are just giving their opinion based on his stats when you check the goals Gyökeres have scored and how consistent he's been with it, that's enough for any fan to get exited and hype such player, the fact that he's even got experience in England before moving to Portuguese League is enough to trigger the hype. Anyways it would cause lots of pressure on the player especially if he fails to live up to the standards the fans want him to be he'll get lots of backlash from rivals fans if he doesn't score up to 20+ goals, that's not something new it's the norms in the Premier League, especially when a heavy amount is spent on a particular player.