That's a bit hyperbolic, is it not? Forest, of all teams, managed to catch up Arsenal before us. Liverpool overtook them without signing a single player! Gyokeres isn't going to transform them. He'll improve them, but where Arsenal have struggled, Gyokeres doesn't exactly excel. He does a lot of his best work with space to run into and down the channels. He hasn't played against low block teams, from what I can see, so I wouldn't be sure he's as transformative as people think.
Well they haven't exactly caught up with them since Arsenal are 5 points ahead and will almost certainly finish second. But I'm glad you brought up Forest, since they are a good example of how a club in 17th can rocket up the table to 3rd in less than 12 months with quality coaching and recruitment. If we get our striker and a solid #10 and/or wide player like Cherki then I'm 99% we will be in a similar predicament next year, especially when all our injured players are back.
On the other hand, if Gyokeres goes to Arsenal - a team who are well coached and have excelled over the past 3 years without a proper striker, then you are probably looking at a club that will go on a league run over the next few years. There's no hyperbole in suggesting a team that finishes 2nd every year would probably win the league by adding a prolific striker.