I looked into purchasing that same Actron gauge when I was having some issues several years ago. You just need a $9 Actron adapter PN: 180-000-1299, Right Angle Large Schrader Adapter. I never ended up buying one so I cannot confirm if it is correct. Info here:
https://www.renntech.org/topic/37106-fuel-pressure-tester/
ike84, can you link to that custom adapter, I would like to take a look at it. Thanks!