It's worth noting that Roshi resists soul manip via
Ki manipulation, which helps him a lot here. Gerson is 2.4x stronger, which is a notable advantage, although a charged-up Kamehameha might be able to surpass it. In terms of raw combat skill and experience, Roshi clears pretty easily, which will aid him a lot in avoiding Gerson's Danmaku on top of his acrobatics and better Analytical prediction. Green Mode prob won't be too much of a problem for Roshi since he can block the arrows with his staff and is skilled enough to put up a good defense against them. Gerson doesn't really have a way around things like Roshi's
Paralysis Art,
Lightning Surprise Attack, and
Hypnosis Technique, which are gonna take Gerson out if Roshi successfully uses them.