This applies to taxis hailed on the street (public hire).
A taxi booked via Gett becomes private hire. Gett do say somewhere additional fee’s may be levied. Can’t remember the exact wording but that covers them in instances like the OP.
My company were using Gett for a bit but their availability/service was sporadic so we switched over to an account with Glasgow Taxis. There are no boundary charges with them. It’s metered rate within Glasgow and fixed prices for airport and out of town.
Edit: screenshot from Gett app
https://preview.redd.it/vsggdws9m2ib1.jpeg?width=1284&format=pjpg&auto=webp&s=b8cd83e43f500cc17b271f4379950acaa92f0c45 |