How should I wall mount a TV with one metal stud and a gap between the drywall and exterior block?
I would definitely take advantage of the block wall. I would use 2-1/2" tapcons with 1/4" fender washers straight through the drywall and than through some type of 7/8" spacer, and finally into the cinder block. The 7/8 inch spacer will prevent the screws from collapsing the drywall as the screws are tightened.   It would be best to use the largest TapCon fastener available. I believe they market a 1/4 inch diameter screw that has 3 versions: Flathead, Roundhead and Hex-head (hex's are easiest to grip and drive). The 1/4 inch screws will need a 3/16 inch concrete bit. I would use 4-8 of them depending on TV weight.