Mod can be built using the gradle command line
gradlew :fabric:build -Plaunch_mod=c
gradlew :forge:build -Plaunch_mod=forge
If you build fabric and then want to build forge, or the other way around, run gradlew clean to clear the cache out so the jar does not contain files meant for the other platform.
Discord https://discord.gg/GZjtAn3F6W