I concur, however believe its generally accepted a unique being is presumed to be the sole example of its race (ie Data was considered unique in the ST universe until the discover of Lor revoked that status), thus the Questioner's Balor is a specific individual but NOT unique and thus required to answer the Gate's calling function.
However not is all lost should the Questioner's GM disagree and allow it to ignore the Gate's call. Simply recast to open a two-way portal to the current location of that specific balor - for while the spell doesnt state you can specifically target a location on the desired plane, doing so seems reasonable given the spell's second function requires the ability to lock onto a specific being. (Note: His GM may prefer the Gate's two-way portal function since the Questioner could return home happy at having resolved his balor problem only to find his casting locale occupied by uninvited Abyssal guests!)
Either way, Gate puts the caster into position to Plane Shift his balor as desired.