Paul's Wish for the Troublemakers

Galatians 5:12 NASB
Would that those who are troubling you would even mutilate themselves.
In the NKJV, this verse reads:
I would they were even cut off which trouble you.
The NLT version reads:
I only wish that those troublemakers who want to mutilate you by circumcision would mutilate themselves.
Is Paul saying that he wishes these troublemakers would circumcise themselves? But aren't they already circumcised, seeing as they're insisting that circumcision is necessary for salvation?
So, Paul must mean something more than circumcision. I looked up the Greek word he used in this verse in Strong's Exhaustive Concordance. Apokopto (#G609) means "to amputate; reflexive (by irony) to mutilate (the privy parts), and it's translated "cut off."
Among the cross references for this verse in my NASB was Deuteronomy 23:1. Read it to find out what amputating that particular organ would mean, according to the Law, which these troublemakers want to obey to the letter.
Reread Galatians 1:8-9.
How is what Paul is saying in these verses similar to what he's saying in 5:12?
How serious is preaching a different gospel?