There are several ways to install mods for Outward. For the GoG version of the game, you have to first start in the GoG galaxy launcher and convert the game to the mono installation. Epic Games has this enabled by default, and on the Steam version, you go through the beta section. "Properties > Betas > "default-mono - Public default branch (mono)." For the GoG version of the game, you have to click the options button, which should be from the right of the play button. Then you should go to Manage Installation then to Configure, then to the Installation menu. Here you should be able to find a button to set the game to the default-mono version. If you see further options make sure to select the highest number for the latest version of the game.
After you have your game set to mono then you have to find some mods. I would recommend using Thunderstore due to the fact that it has a built-in mod launcher as well as having more up-to-date mods. It is highly advised when modding to make a backup of Outward\BepInEx, Outward\Mods, Outward\winhttp.dll, and Outward\doorstop_config.ini. These files are used in a ton of steam games and making sure you have a backup no matter what the game is good practice. Next download r2modman, the mod manager that Thunderstore uses. Choose Outward as your main game, and create a profile for the launcher. Make sure your installation is correct by checking: Settings > Locations > Change Outward directory. Last but not least you download BepInExPack Outward pack in the mod database and any other mods that you please.
The only thing that you must do to play modded is to launch from r2modman, if you want to play vanilla you can either click that option or launch directly from your preferred launcher. There are other methods of modding the game but using r2modman is typically the most reliable.