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All at once? Gallantmon will probably lose. He could probably take all of the pokemon 1v1 (probably 3,4, maybe even 5 legendaries at a time), but just all of the legendary pokemon together would probably overwhelm Gallantmon. Digimon are strong, but 649 against 1 is just too much.
"The world's made up of numbers. Do the math and you'll find your desired solution." -Sho Minamimoto