Well, Gai kind of doesn't need hand seals..so as long as he can keep up with his swords play he can enter 6 or 7th gate and continue to blast Mifune. But contrary to popular belief I think it could go either way
mifunes faster. gai wasnt able to speedblitz swordless kisame and stop him forming seals. mifunes stronger. he cut a large summon in half and broke hanzos kurasigawa