GDAL does not recognize the file as ERDAS imagine, as promised by the extension
.img. But it looks only at the file extension, and does no further investigation on the content of the file.
Comparing with other data from the site, they seem to provide unformatted raw binary.
The mask should be 17347 lines by 40031 samples (columns)
So I created an ESRI hdr file for it with this content:
(actually I took a SRTM file, exported it as hdr and changed the size definitions)
Renamed the .img to .bil, and created a .prj file for the used Goode Homolosine:
PROJCS["Interrupted_Goode_Homolosine",GEOGCS["GCS_Normal Sphere (r=6370997)",DATUM["D_unknown",SPHEROID["sphere",6370997,0]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Interrupted_Goode_Homolosine"],UNIT["Meter",1]]
And got an image that fits quite good to Natural Earths vector data in green: