Apologies for necro-posting, but since this is the top google result for "forge gradle exclude xcf files from jar", I thought I should put in my solution.
You can exclude using sourceSets, like this
sourceSets {
main {
resources {
exclude '**/xcf_folder_name/*'
exclude 'resources/assets/<modname>/path/to/xcf/folder/xcf_folder_name'
}
}
}
So, for my mod (Minechem), I use this:
sourceSets {
main {
resources {
exclude '**/xcf/*'
exclude 'resources/assets/minechem/xcf'
}
}
}