If we make a few (wrong!) assumptions:
1. Avgas has a chemical formula of C8H18.
2. It is 100% oxidised to CO2 (clearly not according to the above, but some of the emissions of partial oxidation are greenhouse gases as well, so let's go with this assumption for now).
3. You burn 30 litres / hour.
You will emit 66.7 kg of CO2. Carbon offset companies would charge you about £0.50 to offset that. Whether that actually does any good or not is open to debate - I suspect that there are a few carbon offset companies that have unscrupulous and rich shareholders.