Guaranteeing Resilient Installations for Defense Act of 2021 or the GRID Act of 2021 This bill requires the Department of Defense (DOD) to implement a two-year pilot program to address vulnerabilities at critical defense facilities and their associated defense critical electric infrastructure. DOD is authorized to make grants, enter into cooperative agreements, and supplement funds to support mitigating actions that address such vulnerabilities. DOD must select at least three military installations that are designated as critical defense facilities to carry out the pilot program. The Government Accountability Office must conduct a review and report on the pilot program.