Depending on Windows version there is a limitation for pathnames. Typically 260 character for the fully qualified file name with the directory name less than 248.
Some of the tools used by GMS might further restrict lengths.
To reduce the potential for path limits being reached GMS creates virtual drive mappings, points them at the project folder and then references the drive instead of a lengthy file name and path.
Occasionally, there have been reports of these virtual drives not being cleaned up after a build. Should that be the case you can remove the virtual drive with the subst command.
subst drive: /D
ie
subst z: /D