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. Such funds must be treated as a reprogramming request and may only be obligated or disbursed after the National Guard Bureau submits a written request and receives authorization from Congress. The National Guard Bureau must annually submit an account of the use of funds disaggregated by appropriation and subactivity group.