This will be a big problem for Arsenal, losing Gabriel Jesus will have quite an impact on Arsenal, considering that he became an important player in Mikel Arteta's squad, before moving to Arsenal Gabriel Jesus played at Manchester City, but his quality has not developed much
Only this season has he appeared as a key player in the Gunners squad, even though he has only been with Arsenal for one season and has made a real contribution to the club. So far Arsenal are seriously monitoring Juventus player Dusan Vlahovic, although last season Arsenal failed to reach an agreement with him, Gabriel Jesus' injury revived Arsenal's interest and the opportunity to bring in Dusan Vlahovic is also quite open for transfers this winter, moreover there is no suitable replacement for Gabriel Jesus in Mikel Arteta's current squad.
I am guessing that it will definitely be big hit for them, but they are a big club and should figure out a way. I know it is always hard to lose a great player and keep the success up, many teams end up losing after they lose a big player, but that is not really impossible to stop, it is something they could do later on and at anytime they could.
This means they could transfer a striker if they lose Jesus for good this season, or if he comes back then they could replace him with some formation change instead of just player change as well to make sure his missing is not a big deal. All in all, they have to endure somehow until the end of the season.