You need to grind on online PvP in order to unlock PvE content, such as bosses rematches and items.
[The game is now patched to the latest version - 1.3.0. We’re sorry for the delay. - GOG] The GOG version is outdated, lacking the latest update (1.3.0) that was released on Steam, which implements a lot of bug fixes, balance and new features to the game. Therefore, the GOG version is inferior to its Steam counterpart. I contacted Gearbox and BBI asking when the patch would be released on GOG and both of them told me that I should ask this from GOG, because GOG was responsible for the development work for the GOG version. I asked GOG through a support ticket and they didn't say a single word after 1 entire month. Unacceptable stance from GOG. I don't recommend to buy the Steam version either, because even if GOG is truly responsible for the development of the GOG version, it baffles me how BBI and Gearbox have no interest to ensure that their game is updated on all platforms that they released the game, so, they're at fault here too.
