If your gedcom import creates an error file, it will tell you what gedcom lines are not imported.
It is possible to edit a gedcom and effectively merge the middle names with the given name field. See Jim Richardson’s reply and Dave Sellars reply.
I think the middle name field, if present, will appear immediately after the given name field. If you look at the file you should be able to verify this. In theory, you could add the middle name to to given name field, and then remove the line with the middle name field.
When you save the file, the text editor will likely add the txt extension. If it does, change it to the ged extension or your software will not recognize the file.
A word of caution, if you inadvertently change other things you may cause fatal errors in the gedcom. Keep a copy of the original, unedited gedcom just in case.