My first go-to is always a pipe wrench - I don't care about the old fitting. That's worked maybe 15 of 18 times. Two of the others I went with a bench vise. The last one failed the pipe wrench, bench vise, and filing down the edges of the fitting tests. The end of the CGA fitting finally just corkscrewed and I gave up on it.
For the record, the one failure was a VTS400, which was old. 30 years? 40? Maybe more.