Tropical maritime trading hub on a defensible island, formerly a pirate base. There is the wizard's guild and temples to pantheistic gods and core races. A Cthulhu cult influence. Lots of corrupt urban politics.
So full D&D integrated Pirate/Maritime Trade city with Mythos under the surface.
I would say it's "medium easy" to plug into a campaign world.
Freeport does make a number of assumptions regarding the presence of gods and elder evils, so you might want to read up on that when you integrate your own pantheon(s).
This basically only means that it's not a good fit for a monotheistic history Europe type of game.
Anything from Dark Ages Cthulhu via RuneQuest and Warhammer to full-up D&D would likely function if you're willing to work for it. (You do know the setting is/was popular enough to receive official conversions into several fantasy game engines?)
Freeport's open gods are all generic placeholders so those of a specific campaign can be slotted in: god of knowledge, god of the sea, etc.
Here is the player backgrounder I did for a Freeport campaign here a while ago Death in Freeport Info thread. I had some things like the gods and outside continent specific things and the Athenaeum be from my own homebrew mashup campaign setting.
Its a port and its free, what more does one need to know? Kidding aside I bought the 3E setting when it came out in the early 2000s. I think it was the softcover by AEG, the first one in that iteration. I took bits and pieces from it here and there and dropped them into a FR campaign with no modifications. There were rules for firearms too IIRC. I did not use the setting wholesale in its entirety, disregarded the history and cosmology, just used locations and NPCs, maybe some seafaring rules. I know this answer is somewhat vague but one thing I do remember is that I wasn't disappointed having bought it and did get some use out of it, so Im going to guess the PF version is similar.