Garp is not equal to Sengoku, he is stronger.
I view them sort of like Roger and Rayleigh in power gap.
Garp is equal to sengoku or slightly superior
Sengoku fought 1v10 against bbp for a while, before Garp came to help
Both took down Shiki
Both also rivaled Roger even tho Shiki was Sengoku nemesis and Roger and Garp had bigger rivalry
You can't view it as Roger and Ray since Sengoku is the FA and also has CoC, Garp has no rank over Sengoku
Stats wise : mythical zoan Sengoku physical stats > Garp