Guarding Readiness Resources Act of 2021 This bill modifies the treatment of funds received by the National Guard Bureau as a reimbursement from a state or other U.S. territory (e.g., Guam) for the use of military property. Specifically, the funds must be credited to (1) the appropriation, fund, or account used in incurring the obligation; or (2) an appropriation, fund, or account currently available for the purposes of which the expenditures were made.