You would go for Gas Seal when your Pump has Cofferdam. Cofferdam is normally used in Vertical Pumps for cryogenic application. It converts liquid to gas to prevent icing on seal area.
Gas Seal also is used for gas compressors where wet Seals will surely fail due to dry running.
I hope it helps.
Plan 61 has throttle bushing that works as safety bushing. This will limit the leakage when the seal failed. It has drain port where heavy leakage will flow instead of splashing to the pump or worker.
Throttle bushing is also called safety bushing. This reduces the leakage going to atmospheric side of the seal when the seal failed or when using quench fluid. You can check API 682 for the specific clearance per size, type and temperature.
Normally, N2 quench will be kept open even during standby period. It helps quench out minimal weepage from seal faces to avoid accumulation under the seal. This could build up under the seal faces and solidify. During start up, the solidified product at seal faces ID or OD may break the seal...