GanttProject supports the import of plans from Microsoft Project software (*.mpp) and as thus is the obvious and most likely only choice for MPP to GAN conversion as *.gan files are only produced in GanttProject.
Convert Microsoft Project plan to GanttProject project plan file.
We have found one software suitable in our database for this conversion.
Multiplatform software - convert mpp to gan on any platform
A freeware project scheduling and management program
Convert from mpp file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.