Maybe you already knows this, but I'll detail anyway.
During Peter David's run, Banner was physically separated from the Hulk and, some time later, they had to be merged again because both were dying. To do so, the Gamma Base developed a nutrient bath where both Banner and Hulk would be submerged. An accident happened (General Ross tried to sabotage the process) and Rick Jones fell into the bath too.
As a result, Banner turned into the original Grey Hulk and Rick became a green, much weaker, Hulk.