I immediately looked this up and indeed for the modern era they have never met because in the records I found they had played in the Champions League in 1972/73 and indeed I also did not know if I did not look for it because I was not born at that time 
the h2h is not really applicable because it was half a century ago and not important to be used as a benchmark.
This is a new era but even though Galatasaray are consistent now and have the advantage of playing home I don't think the result will be too different because the possibility for Galatasaray is only a draw in this match but even then it is difficult because it is very rare for Bayern in the Champions League especially for the group phase they get a draw because they always sweep the victory in this group phase.
So I remain confident that 3 points will belong to Bayern.
After reading your reply I also became curious and traced the match between the two clubs and it was very far away and cannot be a benchmark because 100% of the players have all changed and if we talk about chances of winning I still hold Bayern Munich to win because in terms of quality performance My players still see Bayern Munich as superior and I say this because I always see and compare the performances of the two clubs which are very clearly different.the h2h is not really applicable because it was half a century ago and not important to be used as a benchmark.
This is a new era but even though Galatasaray are consistent now and have the advantage of playing home I don't think the result will be too different because the possibility for Galatasaray is only a draw in this match but even then it is difficult because it is very rare for Bayern in the Champions League especially for the group phase they get a draw because they always sweep the victory in this group phase.
So I remain confident that 3 points will belong to Bayern.
But even though the quality of the players is different, I think Galatasaray has an extraordinary defense, even though they can't score, I'm sure the club's defense will make it difficult for the Munich players to score and end the match in a draw.
To bet on this match it looks like I am still doing my research and maybe I will take a double chance for this match.
But it would be better if Galatasaray tried to match the intensity of Bayern with a fast game than using a defense scheme but indeed this would be more draining but the results could be more benchmarked.
Copenhagen did the same thing before and they managed to hold Bayern just that lack of focus in the crucial minutes made them have to give up 1 point in the last minutes. Galatasaray can also try that, it's just that the risk is clear when trying to keep up but it doesn't go well then this can backfire for Galatasaray itself.