Galatasaray will play in the UCL and then they will have some tough league games ahead against the top name teams as well, it is not going to be easy for them. The game against Fenerbahce will be the most important game of the season for Galatasaray, because Fenerbahce wins then there is no points difference and equal points whereas if Galatasaray wins the gap will be six points and Galatasaray needs it because they will definitely lose points without a doubt.
We of course don't know how Fenerbahce will do, we could see them not do as well as they have done so far neither, they are the ones chasing after all. UCL and Europa means a lot, because players needs to be fresh and rested, otherwise league will become an issue.
If Galatasaray passes the next round in Champions League, they will play european matches before and after the Basaksehir match. if they are still ahead of Fenerbahce after these 6 matches winning the league will be very easy. But if they loose more than 3 points, winning the league will be very hard.
Galatasaray won Juventus yesterday with a big goal difference, and from the look of things Galatasaray is halfway to qualifying for the Champions League. Turkish league matches are remaining twelve matches for the season to end, so I agree with you that the league will be easy for Galatasaray to win if they play six matches and still be ahead of Fenerbahçe. Because I don't see Galatasaray dropping points that will give Fenerbahçe a chance to overtake them in the table and win the Turkish league title.