I found the projection on the GRUMP FAQ page[0]. Turns out they don't use a projection:

    *Projection: None, the data is in Geographic (latitude/longitude)*

and so the resulting map is similar to the Mercator.

I wonder how easy it is to map lat/long co-ordinates into different projections - would be a cool option to have on all similar maps.

[0] http://sedac.uservoice.com/knowledgebase/articles/41635-what...