Gmap Views - Proximity filter for user locations based off of a specific node location
This is really a Views question. You can specify a Relationship and a Contextual Filter under the Advanced tab in Views.

I don't know the specifics for Gmap Views, but the principle is the same: You'd make a view of users, with a contextual filter of content:NID. Then you'd create a relationship to nodes based on the location. Gmap Views might give you a way to say 'within x miles of'... This is Views' way of creating a JOIN query.

Then on your content you'd add a Viewfield and tell it to pass the NID to the view. This will attach the view to your node and generate the view of users nearby.

Helpful tutorial about relationships here. Start at about 1:00 if you're impatient. :-)