Yes. Although the GI Bill covers much of the cost of attendance, it is important to apply for FAFSA because you may be eligible for federal grants that you are not required to pay back. You may also be eligible for subsidized and unsubsidized loans, which will have to be repaid. Make sure to discuss this decision with a financial aid officer before accepting loans.
Visit https://studentaid.gov/ to apply; the deadline is typically June 30 for fall semester entry. You will need to file the FAFSA each year of your postsecondary education. State aid may also be available. Talk to the admissions or financial aid office of the institution you are attending for guidance.