For sure his presence will be felt in the team because he has been outstanding in the squad this season and made very positive impact in the teams progress to becoming title contenders and I will love to see Arsenal continue in that tempo and spirit after the break. Gabriel Jesus injury is a huge blow to the team as they need enough force to compete with Manchester City. Arteta knows whats at stake and I believe they will get a striker in January to fill in with Nketiah until he returns.
True, Even from goal productivity he is still a little lacking but seeing the movement and shifting focus of the defenders against the real opponent Gabriel is very good and will be a miss indeed for Arsenal at this time so there are some rumors saying about the possibility they will look for other alternatives in this regard.
Gabriel will undergo a long recovery because at least 3 months is said to be normal for a complete recovery if there are no other disorders.