Well, aside from Gyokeres known to have played for a mid-level team like Brighton in the EPL before leaving for Sporting CP, he was also younger at that time and didn't had the experience to make any much of a difference in the attacking front for Brighton as at the time, unlike now he has gained much experience with age over the years in Portugal. And being knowledgeable about how the EPL is as someone that has played there before, it is very possible he will shine at Arsenal scoring competitively against Haaland with all the balls he will be able to get from both the midfielders and wingers in this Arsenal team.
Gyokeres failed in the past but succeeded in Sporting Lisbon and now he has a second chance to show his ability in Premier League and with a big club Arsenal. I don't let past performances of a player especiallly when he was very young affects my thinking about him in the present. Don't forget that when Mohamed Salah was very young, moved from Basel to Chelsea, he failed there. The same with Kevin De Bruyne and both Salah and De Bruyne later grew up amazingly and became one of best attacking players in many years.
Gyokeres has talent to do the same while Arsenal is a strong club last three seasons so they can mutually support each other and aim at a successful season. Arsenal lacked of a world class striker in latest years and Gyokeres potentially can help them to fix this weakness at a position which can contribute many goals for their club.