Garage floors aren’t sloped by design.
On houses with attached garages the floor of the garage is surrounded by the foundation of the house, thus they can’t slope it any noticeable amount due to the basic tenets of construction.
I dont really know why you would want it sloped anyway, i mean the whole water rolling out thing is a fantasy anyway, and if the sloping wasnt done perfectly right the water would all pool up in the front corners by the garage door rails, causing them to rust or keeping the bottom of your door wet.
As for the home gym, best place is a ground floor, be that garage floor or a basement with a decent ceiling height. If you put a good home gym into a floor above ground level you should probably put some extra plates of plywood down under it all, assuming you plan on lifting some good weight. Then put those rubber mats on that to deaden/soften the blow from dropped weights.
I dont know how you guys who live in northern climates can handle giving up your garage space, I hate cleaning the car off in winter and shoveling around it. Heating the garage would also be required, and cooling it in summer.