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.
You have a point, but my understanding is that Sporting has issues with the structure of the add-ons... They want all of the conditions to be very easy so that it'd guarantee them receiving the whole add-ons amount, but Arsenal only want to make it partially easy
[having said that, I'm a bit surprised it's taking this long to come to the final agreement when it has been very close in the past few days].