Games from Space increases your GPU utilisation. The GFS app always kicks in when your GPU otherwise would be sitting around doing nothing.
To power your GPU of course there is electricity consumption and with GFS this consumption is also increased to some extent.
The actual impact differs largely from user to user as you need to factor in: type of GPU, GPU wattage, GPU utilisation as well as the actual cost per kwh.
Information about your GPU's rated wattage can be found on the manufacturer's website, while your cost per KWh is found on your electricity bill.
Getting an estimate
To get a good estimate you can use online calculators (eg. Calculator for US by energy.gov) by inputting the max wattage of your graphics card (which you can usually find on the manufacturers website) and inputting the amount of hours you are intending utilise GFS per day.
Keep in mind that by using the max GPU wattage in calculations, this will calculate your energy cost assuming that the GPU is running at 100% for all the hours you are using it. If you are not running GFS at max 24 hours a day, or you are running GFS in Medium/Slow, your actual power usage may be much lower.
For the super accurate
If you want to be absolutely sure, you can buy a cheap wattmeter online. You can get a decent one for under USD 10.
(If you do get a wattmeter we would love to hear about your findings - please reach out to us via Discord!)