Agreed. BTW I think the Subie part is Holt's Stop Leak relabelled. If it was me, I'd check that it definitely is the gasket. If so, you're probably stuck with fix or dispose, but just in case, I'd do a full system flush at home, treat it with a chemi-weld type product & then refill the system using the Subie cooling system conditioner. Then if you still have to do a head gasket, well at least you tried some options.
Actually, if the gasket's gone and it's been losing coolant &/or overheating for some time you oculd wind up cheaper swapping the engine for a good s/h one in case there's additional damage. Cheers