There are 12 outfits in total, including the original outfit.
A new outfit is unlocked for every 3 special gears found.
Hence you will need to find (12 - 1) x 3 = 33 special gears to unlock this trophy.
There are 45 levels in this game, and a special gear in every level, getting 33 out of 45 levels shouldn't be too hard.
But consider it takes 10-30 minutes for each level, and you might not be able to obtain the special gear in the first go, this bronze trophy may cost 10+ hours to earn.