Guaranteed Income Pilot Program Act of 2021 This bill directs the Department of Health and Human Services (HHS) to establish and implement a three-year pilot program to provide a guaranteed monthly income to individual taxpayers between the ages of 18-65. HHS must consult with the Internal Revenue Service and certain nonpartisan and nonprofit agencies or academic institutions with expertise in social science experimentation to develop and award income subsidies to eligible taxpayers and to conduct a study on the outcomes of the program.