Hello SiuP
First of all: welcome to the SBB Community.
Public transit tickets in Switzerland are always valid issued for a route, not for a specific train company.
Any company can sell any ticket, it is all and the same ticket vending system, no matter where you purchase.
In that regard, yes, SBB can sell tickets for the route the Glacier Express is serving.
You need a ticket and a seat reservation, each for the corresponding travel class.
Kind regards,
Roland