It's a small, jij-able library that allows creation of server side guis.
Add it to your dependencies like this:
repositories {
maven { url 'https://maven.nucleoid.xyz' }
}
dependencies {
modImplementation include("eu.pb4:sgui:[TAG]")
}
After that you are ready to go! You can use SimpleGUI and other classes directly for simple ones or extend them for more complex guis.