Taking down GSA would largely cripple the remainder of fedgov; they supply everything from photocopy paper to construction of office buildings. Every leased building that the government uses is leased by GSA. Most of the federal motor pool fleet vehicles are likewise obtained through GSA. It is a massively powerful agency, due to the broad scope of its involvement in other agencies' business.
Some here may remember the Las Vegas scandal from early in Obama's first term - GSA blew over $800K on a "training seminar" there.
As you rightly point out...they are in every agencies business.
I appreciate the economies of scale, but each agency should do it’s own landlord and support services.
The GSA is TOO big and is a major reason for the shitshow that is the FEDGOV