By the OP’s photo, the gage brass threads look smaller in diameter than the circled thread, but I could be wrong.
It looks like the gage threads are 1/8-27NPT and the circled fitting looks like 1/4-18NPT, again, I could be wrong. Measure the circled fitting with a caliper or eyeball it with a scale (ruler). If it’s a 1/4-18NPT it will be about .540 diameter at the largest point (the thread is tapered so it will get larger as it gets farther from the end that’s screwed into the fitting. If it’s a 1/8-27NPT it will measure about .405 diameter at largest point.