The requirements found in 210.8 of the NEC has no bearing on what the manufacturer of the treadmill recommends.
The NEC mandates that all 125 volt 15 and 20 amp receptacles installed in a garage be GFCI protected and we must adhere to this rule.
If the manufacturer of the treadmill recommends that the equipment not be plugged into a receptacle that is protected by GFCI then I would suppose that it couldnít be used in the garage.
If the treadmill is tripping a GFCI then there is a problem with the treadmill that needs addressing. UL has mandated that no appliance have more than .005 amps (5 milliamps) of current leaking to ground for many years now so yes indeed if it trips the GFCI it does not meet the Standard by which UL mandates and this alone says it has problems.
I have a very old treadmill that I walk on four days a week per doctors orders. This treadmill is plugged into a GFCI receptacle and it doesnít trip or at least it hasnít for the past couple of years. The minute it does--- out the door it goes!!!!