I certainly would not let it destroy my day as I would go for a run nearby.
I agree that in the majority of races GPS would probably not have a massive competitive advantage but in a pea souper with horizontal rain blowing in I think a pre loaded GPS route might have an advantage. I was not at the Mourne Highline but understand that use of GPS certainly had an impact on the results and have spoken to one runner who was there who has not raced since.
I'll repeat my point that if GPS is to be allowed in any FRA race then that race MUST advertise the fact prior to the event so that all runners can make an informed decision whether to take part or not.