Any arm will sound lovely on a Garrard - it's just a motor unit, and as such it's scope is only limited by the physical plinth size and structure. The 'matching arms to decks' thing really only applies to small suspended subchassis decks like say a Linn, Gyro etc - these can't take a long or heavy arm as it wont physically fit or would unbalance the deck's suspension. With a Garrard, SP10 or any other proper motor unit you really can do whatever you like: 9", 10" 12", low mass, high mass, air bearing parallel trackers, more than one arm, anything! Nothing is 'too good' either, e.g. want a Triplaner, Ikeda, Breur or whatever? Then go grab one. Grab two or three if you like / your plinth is big enough.
I suspect much of the choice seems to be based around aesthetics, and nothing looks nicer than say a SME 3012 or Ortofon RM309 on these vintage decks, but that's missing the point - *any* arm will work fine. Just make sure to choose it in combination with the cartridge - it is the arm / cart that needs to be mathematically correct when it comes to mass / compliance etc. The 401 just spins the record around.