Their throat and belly are dark rufous and the undertail coverts are gray with white or buff edges.

[7] The grey-tailed mountaingem is found only in the Cordillera de Talamanca of southern Costa Rica.

Its nest has not been described but is believed to be similar to that of its close relative the white-throated mountaingem sensu stricto.

The grey-tailed mountaingem makes high pitched calls described as "ziit or ziip" and also "a 'sputtery, bubbly' song".

[7] The IUCN has assessed the grey-tailed mountaingem as being of Least Concern, though it has a small range and its population size and trend are unknown.