According to the mod's description, it's supposed to prevent certain mobs from freely moving through a fence gate, but able to do so if the mob is on a lead. However, even on a lead, I am not seeing that behavior.
The following is our server config for the mod. I would assume that removing those mobs from the block list would let them walk through whenever.
#The default behavior for entities trying to pass through fence gates.
#
#Possible values:
# ALLOW - Entities may pass through fence gates by default. Exceptions may be added to the blocklist.
# BLOCK - Entities are blocked from passing through fence gates by default. Exceptions may be added to the allowlist.
# CHECK - An algorithm is used to determine if an entity should be able to pass through fence gates on a best-effort basis.
# Exceptions can be added to the allow- or blocklist to respectively allow entities to or block them from passing.
#
#Allowed Values: ALLOW, BLOCK, CHECK
defaultBehavior = "CHECK"
#Entities that are always blocked from passing through fence gates.
blocklist = ["minecraft:cow", "minecraft:pig", "minecraft:sheep", "minecraft:chicken"]
#Entities that are always allowed to pass through fence gates.
allowlist = ["minecolonies:citizen", "minecolonies:visitor"]