Group Support Battalion. They used to be company-sized, but have grown as the needs of the Groups have grown.
In the 90's you had something like this:
An SF Bn.:
3 x SF Co. with one B-Team and 5-6 A-Teams. (I forget)
1 Bn. Support Co. w/ 1 Service Det. (riggers, mechanics, loggies), 1 Signal Det. (commo guys), and 1 Intel Det. (All the Intel bubbas). Each Det. was about 30-35 in size
1 HQ Det., your Bn. HQ folks like S-1, S-2, etc.
Then each Group contained 3 battalions of the the above, but at the Group level the HQ element was a Co. and they had a Group Spt. Co.. We rarely worked with them back then, but the gist was they could be plugged in where needed to provide extra support for a Bn.
Much of the above is still true, but the manning levels have changed.
ETA, a bit on a GSB with a badass cover shot.
https://www.soc.mil/528th/PDFs/1st GSB Special Warfare Magazine.pdf