First, graphical editors all have their limitations in handling very large text files. They will choke sooner or later as the number of lines increases. They will choke sooner or later when the lines become longer (try creating a very long single line in gedit).
Therefore, avoid having to deal with such large files for manual edits, and prefer to use dedicated tools to view or edit such large files.
Still, Gedit has come a long way in supporting opening and editing quite large files. For me, it opens a 180 000 lines text file without any issue. For larger text files, a blue dialog is implemented showing the progress. At that point, I also see the issue with the black screen. However, that issue appears not related directly to Gedit, but rather is related to limitations in the the graphical toolkit, GTK.