Per Juvela's comments.
You said you bought off the internet.
Is it a Canadian made GP? If so it might be standard thread. I had a Carlton made Super GP that was Raleigh thread. Not certain but I believe the other 4 I had were Canadian made and that I switched out parts with no issues. Someone more experienced than I will hopefully post to confirm or refute Canadian made threads.
I can check my son's GP if you need.
Update: Digital caliper batteries are dead. May have to reach out to @
T-Mar or
bertinjim for advice....