Is there a gas that can do the opposite of helium to the voice?

Yes indeed. Helium does strange things to your voice because it is much less dense than air.

In your throat it is acting a bit like a musical instrument. You get sound waves vibrating backwards and forwards, up and down above your vocal cords. That gives rich timbre to your voice.

It picks which frequencies of your voice to amplify.

Helium is a much lower density gas than air, and that means that sound travels much faster in it.

Then your throat will vibrate at much higher frequencies. It will amplify the sounds at much higher frequencies.

To reverse the helium effect you'll need a much denser gas than air.

There are a couple of good ones. Xenon would work beautifully, which is a noble gas, very safe. Another good one to try is sulphur hexafluoride. Both of these are much denser than air so you'll amplify the much deeper sounds in your voice.