By that logic that means Lee, Neji, Yamato, Anko, the other ninjas with Anko and Yamato, plus Hinata's dad and Hanabi can all beat the 6 paths solo style too huh? Gai out right said he surpassed Kakashi while going 8th gate yet Kakashi had trouble fighting 1 Pein then even more trouble fighting 2 and still died after getting backup and you think Gai can beat the 6 Paths?
Not only is Nagato a sensor but he has share vision which allows him to not lose sight of his target plus his eyes, the Rinnegan, would no doubt keep track of Gai's movements. Before Gai can even go gates he can be pulled by Deva Pein and impaled by Asura's blade tail that got Kakashi's Lightning clone, or get his soul pulled out, or stabbed with chskra recievers, or asura path use his powerful laser, or the path the summons the hell summon create can pull souls out as well as it did to Konoha ninjas during the invasion.
Youth blitz? Gai needed to go 6th gate to fight a 70% powered Kisame clone then 7 gate to fight a weakened Kisame who didn't have Samehada and you think he can fight 6 Peins who are Nagato who was solo King?