The weapons of choice of the main characters are perfect for their respective characters, even putting aside Goemon's obvious choice of a traditional sword:
Lupin's Walther P38 started out as a cheaper but equally tough replacement for the Luger pistol in the Wehrmacht, and after World War II production continued in post-German Occupation France through captured machinery. In a way, it's a gun the French stole from Germany, fitting Lupin's profession, heritage and the scope of his capers.
Jigen uses a S&W Model 19, a model of .357 Magnum revolver mostly used by the American police forces and developed on request of a retired one searching for a better gun than the Model 27, fitting in with how Jigen is the most "American" of the characters and an experienced shooter who knows his guns.
Fujiko favours pocket pistols (the exact model varying depending on the production), submachine guns and explosives, the former fitting with her sneaky nature and the latter on how she's someone you underestimate at your own risk. The fact she doesn't always use the same gun also fits with her unpredictability, her habit of betraying Lupin and everyone else depending on her interests, and her resourcefulness in doing so.
Zenigata uses a Colt 1911A1. It's the first handgun the Japanese police ever adopted (being forced to arm itself by the American occupation authorities after WWII and supplied with those), fitting with how he's an old cop, and the continued success of the classic design also fits with how this old man can still keep up with Lupin for so long.
In general, all the Lupin gang (and Zenigata, who is obsessed/talented enough to be close on their heels) are varying degrees of being masters of disguise, which can explain in-universe for their fluctuating designs across the franchise's history. Any changes by the Lupin gang and Zenigata are them mixing up their default look to throw off pursuers or to get closer to their quarry, respectively.
