Maybe it has a nominal use as well, but this term of abuse is primarily a causative imperative meaning "Get fucked in the ass!" (lit. "get your butthole beaten"). "Maraao" is the familiar imperative of the verb "maraanaa", which is part way along the causative chain that begins with "marnaa".

marnaa = to die
maarnaa = to cause to die, to beat
maraanaa = to cause to beat
marvaanaa = to cause someone else to beat

The noun "gaaMR" is probably a dialect variant of "gaRhaa" = hole, pit, cavity.