No way to establish cost basis on gifted House

Back story: 10+ years ago my grandmother gifted me her house in order to qualify for a certain nursing home. The house sat vacant but maintained until I sold the house in April. The house was built by my grandmother and grandfather in the 1950s. I can’t not find records of a mortgage used to build it. Both grandparent have passed prior to the sale but was not inheritance.

For tax purposes, since the house was a gift I know I can use the donors cost basis as my cost basis. But what do I do if I can’t determine their cost basis? Do I use $0 and pay tax on full sale amount? Fair market value can be used to step up value for inheritance but not a gift. I’ve read as much tax code as I could handle. Any suggestions?