1. Create a new actor asset.
2. Create a FGameplayTag local variable within a function.
3. Set the default value of the variable to a gameplay tag.
4. Open the reference viewer using the actor asset as the root.
5. Within the reference viewer settings check Show Name References as true.
6. The gameplay tag set as the local variable default will not be shown in the reference viewer.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-199247 in the post.