You shouldn't write any of the strikers off because Arsenal also need a player that is good with the ball and it will be bad that Viktor Gyökeres will be that kind of player that find it hard to battle for the ball and it will look like the Gunners are always playing with a man down, I'm just saying. Both players are good which I'd love Arsenal to get both but where the problem lies is how stingy the Gunners are to spend to get quality players, with time one of these 2 players might change their mind or even move to a rival club if Arsenal ain't serious.
Benjamin Šeško didn't score up to 20 goals in the Bundesliga doesn't mean he couldn't get those figures in the EPL and it could be that Šeško will even score more than Gyökeres if both players plays in the EPL. Arsenal style of play will make any good striker to score more than 20 goals, imagine the Gunners made Kai Havertz get 9 goals when he's not a proper number 9 then what if they had a number 9? It will be more than 20, is best they decide which of both strikers they want.
When you said "where the problem lies is how stingy the Gunners are to spend to get quality players" this reminded me of Arsen Wenger's time as the coach of the club where he sees signing of players with high feelings as a total waste of money. He can actually use £50 million in a transfer window in purchasing 5 - 10 players with that money and teams like City and Chelsea will use even more than that amount to buy one player. At the end of it all, he will be struggling to make top 4 in other to qualify for the Champions League while teams like Manchester City and Chelsea will win the league. I didn't include Manchester United because during the days of Sir Alex they where not spending big at least from 2008 - 2013 that he retired.
Instead they even sold Ronaldo for a hig fee to Real Madrid.