Q. How do I cite in MLA citation style a public law document found online at www.gpo.gov?
I can suggest a very useful site from Indiana University: Guide: Citing U.S. Government Publications.
Check two examples: Public Laws and U.S. Code. The first will give you an idea how to cite properly a public act, the second will show how to cite the document retrieved from www.gpo.gov. Also, following the suggestion from the same site, you will need to add the elements specific to MLA citation style. Check the UHD MLA Citation Guide ("Websites and Databases" tab) to see how to format your citation and add medium and date of access. For in-text reference, check the "In-text Citation" tab in the same guide and follow "Work with no Author" examples.