I had to add a circuit to an old GS panel recently, and to my utter disbelief it actually listed all the non-Square D breakers as being compatible! (GE, Crouse Hinds, Westinghouse, etc) I've never seen a "universal" panel before.
The actual GS breakers were Westinghouse. The colored handles should be a dead giveaway. They even have the same UL "E" number as the Westinghouse of that time period, even though they are branded differently.