Two most excellent points Herman makes. The water coming out of the gauge when you need to bleed air out of the filter is pool water, relative clean (but not fresh water). It'll just squirt out a bit as you tighten back up the valve. The water coming out of the drain plug will be filthy, most of the sludge that collects in the filter. I've written before here about plans to somehow convert that plug to a valve or hose bib, so I could drain off the water to some other part of my yard through a hose, but I've yet to implement that. The nasty water slushes out over my pad and collects in the surrounding river rock border, embedding itself permanently for the most part. Yuck!
In your case, it's going to run out across your patio and slime it pretty well. Had the heater and filter locations been swapped, you might have been better off. Technically, it's not too late to do that.
If I come up with a solution for the drain plug valve idea, I'll share it.