So I went and checked, and found the following:
On Steam, there are two DLCs: one that grants two more hairstyles and two more costumes, and one that's the soundtrack.
The soundtrack is already on GOG.
For the other one, it seems it might already be in the base game on GOG. I checked a completed save file and found that I had both the black dress and the white dress shown in the DLC page on Steam. I attached a picture of the white dress, which you can compare to the GIFs provided on the Steam store page:
https://store.steampowered.com/app/1202080/Tower_hunter__DLC2_Fashion_Package_1/ .
I don't know what hairs the DLC gives me though -- the example images on Steam aren't clear.
But it seems likely that the DLC hairs and costumes are already in the base game here at GOG.
I actually started looking into this because I noticed there were ten spaces for hairs and and ten for costumes but I only have nine each. I don't know if those are just unfilled spaces; they might very well be, since I've finished the game and already seem to have the DLC dresses. Or maybe they're a prize for finishing NG+ at higher difficulties?