Get the Mod Manager
GCMM is the GameCraft Mod Manager created by NorbiPeti which handles all the complicated parts of mods for Gamecraft. Get GCMM by downloading the latest GCMM.zip file from https://git.exmods.org/NorbiPeti/GCMM/releases. When GCMM.zip is finished downloading, extract the zip file into a new folder. Inside that folder, run GCMM.exe to start the application. If SmartScreen blocks GCMM from running, click "More Information" and then "Run Anyway" to override. In extreme cases, your antivirus software may block it as well (unfortunately, I'm unable to offer advice on overriding every antivirus software out there). We promise that Gamecraft Mod Manager isn't a virus, but if you don't trust us run a virus scan on it or read GCMM's source code.
Manual installation is so yesterday
With GCMM starting for the first time, it may prompt you for the location of Gamecraft on your computer. Point it to the location if necessary. With GCMM configured and running, select the mod(s) you want to install and click install. Then launch Gamecraft with GCMM. You will need to launch Gamecraft through GCMM at least once initially, and after every Gamecraft update, for Gamecraft to load mods. After that, you can launch Gamecraft as normal through Steam and it will load mods. And that's it. You should now have a modded Gamecraft client with your choice of mods installed.