Why would Luan be 'clearly a better player' than Mario Gómez? First of all; Luan isn't even a proper striker, he could be a decent winger for Barcelona at best. That doesn't fit the profiel Robert said to be looking for and that doesn't fit the tactics Barcelona are playing. The key for Lucho is to create space for Messi and that's the main reason our little magician is playing from the right. We saw under Tata Martino that opponents could defend him more easily when he's playing in a central role. Luis Suárez opened up space for him and that's why Barcelona is thriving under Luis Enrique. Messi and Neymar need a striker that occupies the center backs of the opponent, so they can exploid the space between the full backs and center backs. That's why the profile of our new signing is a pure striker and not an attacker that can play on the wings as well. We have enough options to be back-up for Messi and Neymar; Arda, Denis, Munir, Rafinha and even Iniesta could play in those positions.
That said, Mario Gómez would be the best option for me. Was one of the best strikers in the world at Bayern, had a less succesful time at Fiorentina because of injuries, but regained his form at Besiktas. If he could be statisfied with the Larsson-role (which means 25 games a season (if not more)), he would be an excellent signing. He provides aireal threat in attack, could be an impact sub in games in which 'the ball just won't go into the net'. Besides that, he would cost only €7M. That way we'd have €15M to add to the budget of next year (which is lower because of the André Gomes signing) and could bring a world class right back to the club.