His main work is about reducing the possibility to extract "rents" from society by closing loopholes in our current economic system:
1. Money is "better" than everything else so those holding money can extract surplus value by lending it out. This was later also found out by Keynes and others. Keynes' solution: inflation. Gesell's solution: imposing carrying costs on cash.
2. Land is required by everybody yet impossible to increase. Solution: the goverment is owner of all land yet leases it out long-term by auction. All income is distributed among all mothers, since the price of land is direct consequence of the number of children/people in a country. Now you would distribute it among all citizens and call it "basic income".
His ideas are still completely valid and deserve a wider audience.