In days long ago in the WTS, Gilead was supposed to supply experienced preachers and teachers to countries that had few jws and not so educated enough to run the branch. As time has gone by, the number of local jws has increased in those outlying countries, local brothers with some education were trained to be COs, DOs, and branch staff. Since sisters cannot be administrators or other such positions, single sister were not needed as much but brothers mostly married to be sure they were more likely to stay in their assignment. These country did not preachers any more, there were enough locally. Only single sisters who came and paid their own way were welcome.
I was told in the mid 80's that if I wanted to get into Gilead, I had to marry a qualified brother and be married 2 years and then apply. Sisters really had no value by then, especially single ones.
Only 3 single sisters went to Gilead that time, and their father was at Bethel, along with several of their brothers. Two married brothers they met in Gilead and one was single for awhile...they had pull you see.