He began practicing as an associate lawyer when he joined his father's firm in 1989. In 1990, he became a trial lawyer at the Economic Intelligence and Investigation Bureau of the Bureau of Customs until 1997. He became Special assistant for law enforcement at the Office of the Commissioner of Customs in 1992. In the same year, he became the chief of Special Anti-Smuggling Unit at the same office. He was a Senior Partner at the Gunigundo-De Leon Law Office. In 1998 election, he became the representative of the lone district of Valenzuela municipality. He won re-election as representative of the second district of the newly chartered Valenzuela City in 2001. In 2004, he lost the mayoralty election amid reports of widespread cheating and election fraud. In 2007 he achieved a landslide victory as Congressman, winning in all precincts and barangay of the second district of Valenzuela City. His fourth congressional term started June 30, 2010, and ended June 30, 2013. Gunigundo is the principal author of several national laws like Republic Act 9048 that authorizes the correction of clerical errors in birth certificates and other civil registry documents without the need of judicial order. The Gunigundo Law shortens the procedure for correction to less than three weeks and does away with the need for the services of a lawyer which is costly and the long litigation procedure in courts that could take between 6 and 24 months before the correction is permitted. It is now easier, faster and inexpensive to get the correction made. He also wrote two books about clerical law and customs modernization. He is also the principal author of Republic Act 10172 that amended several provisions of Republic Act 9048, expanding the power of local civil registrars to also correct errors in the date of birth and gender in birth certificates. Gunigundo is the driving force in the adoption of the principles and framework of the Mother Tongue- Based Multilingual Education as a key component of Republic Act 10533 or the “Enhanced Basic Education Act of 2013”. This law also increased to 12 years the Basic Education cycle that used to be just 10 years. This landmark legislation recognizes the importance of language in educating children and that a strong first language education facilitates learning and proficiency in the acquisition of second languages like Filipino and English. The Philippines has around 170+ languages and is considered to be a multi-lingual nation. The first language of the child will be used as language of learning and assessment in the primary grades. There will be a gradual transition phase starting in Grade 4 wherein Filipino and English shall be introduced as languages of instruction. This new language in education policy following a strong mtbmle model will help churn out better learning outcomes for students in terms of higher participation and completion rates as well as National Achievement Test scores; and enhance the Philippines's capability in achieving Education for All goals and Millennium Development Goals. Upfront costs are estimated to be 1% of the total budget for the Department of Education which will be spent in intensive in-service training of 514,000 public school teachers and production of new teaching materials. This investment is perceived to be recovered in 5–7 years. Hence, it is cost effective since maintaining the bilingual policy using two second languages would be more detrimental in terms of poor learning outcomes and low level of productivity of the workforce that the education system produces. Gunigundo is also the author of RA 10535, better known as the "Philippine Standard Time Act that mandates synchronization of clocks with the time observed by PAG-ASA, the official timekeeper of the Philippines. Effective June 1, 2013, all government offices and media networks are now required to use Philippine Standard Time as a basis in set ting their timepieces. Gunigundo hopes that instilling the value of time will make Filipinos better time managers and consequently more productive in spending their precious time. In 2013, he narrowly defeated Councilor Shalani Soledad-Romulo for the congressional seat of the 2nd district of Valenzuela City.
