A modern, treasure hunting game is not something I would think to run long term, but sounds great for a mini-campaign. And GURPS does really well with modern era stuff, especially in 4e. 3e had a few clunky rules for guns (especially shotguns); 4e has smoothed all that out and made it much easier to run.
Templar's Gold does sound cool. Bizarrely, despite playing GURPS since the '90s, I've never used the gun rules! I have never run or played in a modern game. Time to remedy that.