Developing mod using:
Fabric API: 0.95.0+1.20.4
Fabric Loader: 0.15.3
Forge: 49.0.19
NeoForge: 20.4.83-beta

Log:

Configure project :common
Targeting Minecraft '1.20.4' on a 'JOINED' platform

Configure project :fabric
Fabric Loom: 1.5.7
Found existing cache lock file, rebuilding loom cache. This may have been caused by a failed or canceled build.

FAILURE: Build failed with an exception.

• What went wrong:
  A problem occurred configuring project ':fabric'.

Failed to notify project evaluation listener.
Failed to setup Minecraft, java.lang.RuntimeException: Unexpected IllegalAccessException occurred (Gson 2.9.1). Certain ReflectionAccessFilter features require Java >= 9 to work correctly. If you are not using ReflectionAccessFilter, report this to the Gson maintainers.
Unexpected IllegalAccessException occurred (Gson 2.9.1). Certain ReflectionAccessFilter features require Java >= 9 to work correctly. If you are not using ReflectionAccessFilter, report this to the Gson maintainers.

• Try:

Run with --stacktrace option to get the stack trace.
Run with --info or --debug option to get more log output.
Run with --scan to get full insights.
Get more help at https://help.gradle.org.

CONFIGURE FAILED in 281ms

This happens when trying to launch the fabric client

This is a duplicate of: FabricMC/fabric-loom#1020 and is a Gradle bug.

Make sure to update the foojay resolver plugin if you are using it, if not ensure that your plugin versons are specified in the root project.