Not actually down the loo
I run it through the wall and into the wastepipe from the wash basin through a washing machine type fitting.
The supply water runs through the pre-filters into the RO-unit. There it is split so that about 2 thirds simply wash over the surface of the RO membrane to remove the filtered solids to waste and then out through the 'waste' pipe. The purified water passes through the membrane and out through the 'product' pipe. The unit will make waste water all the time it is running.
There will probably be too much waste for your garage gutter, unless it feeds direct into a drain. Be careful, I thought mine ran to a drain, it didn't, it just went into a 'french' drain - aka a soakaway. After a couple of days my lawn started to flood.
If you have a large(ish) garden you could just let the waste run on to it, that way you can claim a rebate on your sewage charge if you're on a meter.