"gas backup" refers to having a backup plan or alternative option using gas. It usually means having an alternative gas-powered source to use in case the main source of power or energy fails or is unavailable. Full definition
-- Wind and solar with natural gas backup produces electricity at about 200 grams of CO2 emissions / kWh. (windconcernsontario.ca)
Using these assumptions, the cost of a solar water heater with gas backup amounts to less than a dollar extra per month about gas alone. (peakoil.net)
The reason wind is so popular is because it requires natural gas backup and is driving a rapid conversion to natural gas in all sectors. (insteading.com)