During WW1 it was found that pilots were blacking out after pulling out from a steep dive or an extreme turn. At the time G forces were relatively unknown to the fledgling aviation community and many theories were bantered about for many years as to why pilots were blacking out although it was known gravity did play a role. It wasn’t until 1931 that a researcher in Sydney Australia named Frank Cotton figured out gravitational forces on the body and it’s centre of mass.
The G-Suit was designed and built at at the University of Toronto’s Banting and Best Medical Institute in 1941 by a team lead by Wilbur Franks called the Franks Suit. It was a bulky suit that used water to apply pressure to the legs and were used by British and commonwealth pilots during Operation Torch. While the suits were bulky and uncomfortable they did prove effective in reducing blackouts by pilots during extreme turns and maneuvers leading to less crashes and being able to out maneuver enemy fighters. In 1944 a new design was rolled out for use by the Canadian and US air forces but as with the first version it still used water jackets around the legs and pilots found it uncomfortable to wear and still very bulky but again was effective in reducing blackouts.
It wasn’t until after the war that the version that most pilots are familiar with came into use and instead of water used gas to inflate and deflate the bladders around the legs and was way less bulky and more comfortable to wear.