increase the value of your home?

I want to install a ventless gas fireplace how much would it increase the value of my home.

I would hold off on that. If gas prices go through the roof it will be a waste of money and actually bring down the value. If you want to just renovate for the sake of adding value you should focus on your kitchen and bathrooms.
This really depends on several factors not the least of which is how your selling agent handles his presentation of the home. For example: If you live in an area that has ice storms, and there are a lot of times when the electricity is interrupted. Another: Is your heat system, presently a heat pump, then the fireplace if it has a blower should reduce the cost of heating during those periods of very cold weather. Another: This also applies to those persons with a central heat system, the Fireplace even without a blower can make the difference between frozen pipes and water damage and none of the previously mentioned. So the question may be increase in the value of your home or decrease in the cost of repairs if you have a major power outage. I have one, I live in Arkansas, when the 300,000 people were without power for a week, I guess I was glad I had it.