Does federal acquisition reside solely under the General Services Administration umbrella?

No. But GSA Administrator Denise Turner Roth makes a strong case for why it should.
You might describe federal buying as cyclical: GSA take on the bulk, but the amount that agencies go it alone will wax and wane, depending on the nature of the buys, available vehicles and the budget environment. The last decade has seen a swell of agency-specific contracts.
“That’s not efficient for government,” Roth said in a one-on-one interview with Federal Times, acknowledging the trend pushed GSA to re-evaluate its offerings. The result: “You will see more agencies being willing to give up their contracts to come to our solutions.”
Keep reading this article at: http://www.federaltimes.com/story/government/acquisition/gsa-gwac/2015/10/27/centralized-acquisition-gsa-administrator-denise-turner-roth/74678648/