Gai wins mid-high difficulty.
With his speed, he can dodge most of Jiraiya's ninjutsus with the exception of Yomi Numa and oil flame bullet. Yomi Numa can be countered by Gai once he goes gates because going gates produces a lot of heat which will evaporate the swamp. Hirudora casually repels any katon in Jiraiya's arsenal and kills Jiraiya. Overall since Jiraiya starts in base, Gai's speed will be too overwhelming for him to achieve sage mode. Summons can be an annoyance but Gai can easily bypass them through sheer speed and potentially separate Jiraiya from his summons, thereby preventing Jiraiya access in casting his collaboration ninjutsu.
