I don't think he's worth that money at all.
All that has happened that has made his career skyrocket is he moved from a decent Championship team to one that's completely dominating the Portuguese League.
But ask yourselves this question: Outside of the three big teams in Portugal, are the rest of the teams in that league better than, say, 90% of the teams in the Championship? No, not really IMO. I would even go as far as to claim that outside of Sporting, Porto, Benfica, and maybe Braga, none of the remaining 14 teams would manage to earn promotion if they played in the Championship.
Gyökeres would flop hard in the Premier League in my opinion, unless he went to City and Arsenal and even then he wouldn't justify the 100 million price tag. Don't want to sound too harsh but I think he's basically a flat-track bully right now.