I've seen this abbreviation used in the Thuringia region as late as the late 1800s. Yet it isn't on even the rather exhaustive list at https://www.familysearch.org/wiki/en/Month_Names_in_German To my eye, it looks like "gtr".
This particular example is from 1701, but I've seen it used in various entries such that could be September, October, or November.
The best I can think is that it is September, a variation on '7ber', as 'g' is the 7th letter, but that's a total guess.