I prefer to give my money to companies that haven't caved to political anti-gun pressure, and in the process ruin the looks of all the guns they make, and also those which add features pandering to the protectionists but which really end up making the gun less reliable when you might need it most.
WHATEVER.
Short memory of Bill's 10-round magazines, magazine disconnect safeties and lawyer induced 75% of an instruction manual STAMPED on the barrel.
To the OP, my first choice is a Smith & Wesson 686 no dash, dash 1,2,3 or 4, from the used market. I truly believe that it is a (FAR) better deal to buy a lightly used one of these for the SAME OR MORE money than either a GP-100 or a new S&W 686.
In 2nd place is the GP-100.
In 3rd place is a new 686.
The lock on the side of S&W revolvers these days is not really a problem in any tangible way, but it is a large flag telling you that you are looking at an inferior product to the older S&W revolvers that do not have the lock.
The good news is that if you don't feel comfortable buying a used revolver, either a new GP-100 or a new S&W 686 will likely satisfy you very nicely. If you didn't grow up on pre-lock and extremely smooth S&W revolvers, you really won't realize what you're missing if you end up with a new 686 or GP-100.
In that way...
The GP-100 is probably a better value if only the two are considered.