What mapping solution do people use that has a proximity filter?
As of writing this, jpstrikesback's sandbox: Proximity is really solid. I have it running on a few production sites without issues. Once he feels it's 100% it is either going to be released as the D7 version of OpenLayers Proximity or use the new namespace "Proximity" (as it really does not require OpenLayers to function). 

I recently wrote up a walkthrough of how to get this up and running for another question on stack exchange. See Need a simple Google Maps solution; lost in the labyrinth of modules.

As you mentioned GMap was ok in D6, but using OpenLayers is much more robust and flexible. It's worth spending the extra time to get it set up. Good luck