Buying from Government Liquidation IS buying from the US Government. They are the private contractor hired to sell off anything the Government doesn't want. Now with that said, you can still find a lot of stuff on GSA, but it's usually stuff that was reserved by a local Government and used and turned back in. They can still be good trucks but you have to know what you are looking at since many may have been sitting on the back lot of some municipality or fire dept. for a long time. You can run into similar issues with truck on GL, not to mention bent driveshafts form them being loaded and unloaded with forklifts.
In my opinion, you are usually better off buying from a private party. You may pay a little more, but normally the owner has already done all the legwork and fixed the most common issues and put it on the road. Do some research as former military vehicles are not something to just jump into buying.