Is it acceptable to install a gasclothes dryer exhaust vent 1 ft. below the level of the dryer exhaust output?

I have to agree with you. I would not run it lower than exhaust outlet. It has to get a draft going to pull carbon monoxide out and i do not think it will when it is lower.

Every gas exhaust runs up not down so it sounds like a bad idea.

It's actually better to vent your dryer downward. Why? Because any lint that makes its way past the filter will go through the vent. If your vent line goes downhill, the lint will go with it.

The dummies that installed the dryer vent at my house ran it up, then down, then back up, and the dip acts like the trap under your sink, and collects lint like crazy. It's a fire hazard.

Yes, run the vent down and out. It'll be fine. Just make sure it goes down the whole way.

And as long as it vents to the outside of your house, there will be no CO dangers.

As I understand you, you ran the drier vent down under the floor, then outside. If you have any low spots in the horizontal run, it will collect water, as you have the moisture from the wet clothing and moisture from the gas combustion. The lint from the drier will build up in the moisture and be like cement after a while. If I were you, I would consult the manufacturer, as this may void your warranty. You should be able to find them on-line. By the way, "heat" doesn't rise, it radiates in all directions equally. It's "hot air" that rises.

I wouldn't worry about exhaust coming back into the dryer, it's forced out by a fan. Is this a gas dryer? If you have a gas stove or oven it just vents right into your kitchen. If your dryer is electric, there are no exhaust "fumes" at all, just hot moist air.

My neighbor's dryer vent goes striaght down through the floor and under the house to a vent under the porch. There is no problem after ten years.