As of July 2021, 12 states have not expanded Medicaid as permitted by the Affordable Care Act,  contributing to 5.8 million people with incomes below the federal poverty level being without coverage. One approach to help cover people in this “Medicaid gap” would be to have the federal government make Marketplace coverage available to those between current Medicaid eligibility levels and  the federal poverty level. An alternative would be to employ a public option plan in the Marketplace to for the same population. The public option would be a government sponsored plan paying Medicare rates to providers. In this paper we show that a public option that typically pays Medicare rates would considerably reduce the cost of increasing coverage in the Medicaid gap. Using a public option instead of marketplace benchmark premiums would reduce federal premium tax credits for people in the Medicaid gap by about 28 percent. Federal spending for the Medicaid gap population would range from $16.6 to $18.1 billion in 2022 with marketplace benchmarks compared to $11.4 to 1$2.3 billion with the public option, depending on the subsidy schedule. Ten year estimates of federal spending range from $199 billion to $217 billion with marketplace benchmarks compared to $136 billion to $148 billion with the public option.