Receptical tester work on GFCI

Testing a GFCI means that you apply a very high resistance between hot and ground (Specific amount) so that a very low current imbalance is created, IF this results in teh CLICK OF DARKNESS (Trips the GFCI) then it is good.

if not, not good

My tester has a GFCI test button so it does not trip the GFCI till I instruct it to. Works too.

Note. I suspect some outlet 3-light testers, the 2nd Green (Hot-Ground) draws enough current to trip a GFCI at least some models.. Again, this indicates GFCI is working very well.

Should not trip the GFCI. But use a tester with a GFCI test button so that you can test a working GFCI circuit.

Try alternate tester
Try alternate receptacle

I use those all the time with no problem. I suspect you had wet hands or the tester is actually bad.

Could be your protected circuit already has some leakage and the added leakage from the tester was the "straw that broke the camels back". Not an unusual thing to happen.

LOL, I guess I better double check it then. Thanks for the reply.

Yes, it should test the receptacle wiring without tripping the GFCI.
There are models that have the button for testing GFCI.
I would guess that the GFCI is miss wired.