Sporting president is making one mistake, which is going back on his word, otherwise asking for a lot of money isn't really wrong at all. If he just came out and said "hey , I want 100 million, that's it" and that was the last of it, then anyone who paid 100 million would get him, that is even more than right now, and nobody could say anything, Gyokeres is a player under contract and the team decides how much they want, end of story.
The issue was, Gyokeres and Arsenal worked out a deal, where Gyokeres and even his agent is making less, just to make sure this deal happens, and Sporting gave a number, which everyone worked towards, but then, Sporting wanted more, and that's the real issue here. If you give a price, you stick to that price, you do not change it higher when that price is matched, that was the bad part of this deal.
That is the problem nowadays in the transfer market, a price tag will be placed on players and after the owners of a club see how interested or eager other clubs are coming for the player they will start thinking twice if they should increase the pay, is not good at all, that alone kills the relationship between the player in question and the club. Maybe FIFA should make it a rule that one's a price is placed it shouldn't be reduce or increase depending on the player if he's worth it.
I feel Sporting CP are just playing with the player and Arsenal but at the end they'll have to let the player go. What are they even trying to prove? Viktor Gyökeres served the club and has been loyal for all the number of years he spent with the club and now he had to try something else and they are making life hell for him, sometimes loyalty doesn't pay.
On second thought, I think it could be what they (Sporting CP and Arsenal) want we all to believe that the deal is becoming harder and harder and it could go down just to make headline every single day, meanwhile they have finalize the deal, just delaying it for their own interest, it could be and it might be the reason, who knows?