For the Hayward swim pro voyager filter, the threads are the standard 1/4" NPT. The issue is that the gauge diameter is too large to fit down into the recessed hole in the cover. You just need a nipple and coupling long enough to get the gauge above the lid. It's not pretty and elegant, but it does its job. I forget the length I used, maybe 1" plus the coupler was enough.
I went through 3 of the replacement gauges supplied my my pool store that sold me the pool equipment and they all kept failing. I decided to go with the much
superior gauge from TFTestkits.net. Other standard gauges should work also.
Edit: add
a link to show how it looks