When you install Gramps, the default setting for dates is the ISO format, yyyy-mm-dd. You can change that in the 3rd tab of the preferences dialog, which looks like this in Dutch:
I made this screenshot with the date notation expanded, so that you can see the choices that are available. The dialog will have different words in German, but the position of items is the same in all languages, so I bet that you will understand this one anyway.
When you change this, and restart Gramps, most dates should appear in the format that you chose, which means that Gramps knows that they are dates, so that it can display them in the right way. It will then display Mai where I see Mei, and English users see May, assuming that you chose to display (abbreviated) month names, not numbers.
In your case, you will still see dates in bold, in the format that they have in the GEDCOM file. Gramps does this for every date that it didn’t recognize as a date in the GEDCOM file, meaning any date that doesn’t follow the standard format, which is 1 MAY 2023 for today.
If there are not too many dates that are wong, it is easy to retype them in the proper format, which is the same format as you chose for display. If there are many, it might be easier to load your GEDOCM file in a text editor, like notepad, or notepad++, and do a global replace in which you might need to replace März with MAR. You may need to be careful with that, if you also have these month names in notes, for which I assume that you want to keep those in German. GEDCOM month names are 3 letter abbreviations in English, so if your GEDCOM has Okt(ober), you must change that to OCT. Same for Dez(ember), which must be DEC in GEDCOM.
If you’re unsure about the number of non-standard dates, there is a quick way to find them, and that is by choosing the Events category, and sort that by date. This will show all empty dates first, followed by all dates that are wrong, displayed in bold, and the good ones last.
This is for simple dates, and you may also see problems with dates before and after (BEF and AFT in GEDCOM), and date ranges. And again, if there are just a few, you can simply retype them, with the proper keyword(s), in German, which might be bevor for Before, and nach for after. That’s just a guess, because I run Gramps in Dutch, and don’t have the German version installed.
I hope that this helps to get you started, and if not, you know how to find me.
groeten uit Driebergen,