Shadowing should only be done if you really need it. Not only does it prevent GeckoLib from getting downloads on Forge for your mod (and subsequently prevents income from your mod), it also can be tricky to setup. Please only do this if you have a legitimate reason. If you do decide to shadow, consider donating to our patreon!
In order to shadow GeckoLib, simply follow the shadow's plugin documentation. Make sure to relocate geckolib to it's own package.
You can view an example
build.gradle that shadows geckolib here