there arent any instructions on how to compile this mod, nor is it obvious. always errors, furthest ive gotten is e: /Users/dots/Desktop/BetterPortals/build/sources/main/kotlin/de/johni0702/minecraft/betterportals/common/Extensions.kt: (231, 47): Type mismatch: inferred type is Any? but IBlockState! was expected e: /Users/dots/Desktop/BetterPortals/build/sources/main/kotlin/de/johni0702/minecraft/betterportals/common/Extensions.kt: (231, 61): Type mismatch: inferred type is Any? but IBlockState? was expected :compileKotlin FAILED

There aren't any instructions because it should compile just like any other mod which is using ForgeGradle (or when FG is behaving, then like any other Gradle project).
Which commit are you trying to build?

Looks like this bug in ForgeGradle to me.
Try running ./gradlew setupDecompWorkspace before ./gradlew build.

Looks like that bug indeed was it.