Prior to entering politics, Gillespie was a paramedic at Memorial Hospital in York, Pennsylvania. Later, he was the director of safety, security and pre-hospital systems at Memorial Hospital. Gillespie was also a president of the York County Republican Club.
Gillespie was first elected in 2002. He is currently a member of the House Game and Fisheries, Intergovernmental Affairs, Professional Licensure, and Tourism and Recreational Development Committees. He is also chairman of the Subcommittee on Federal/State Relations and Arts and Entertainment.
The 47th District from which Gillespie is elected includes residents of Conewago, East Manchester, Hellam, and Manchester Townships in a northeast portion of York County, their surrounded and adjacent boroughs, and three (the northernmost) of Springettsbury Township's eight local districts.[3] Gillespie received 21.5% of the vote in a crowded eight-candidate Republican primary in 2002, which sent him to that year's general election.[4] While Chad Lonergan had won the corresponding Democratic primary,[4] Gillespie faced only independent candidate Marlin Cutshall in the 2002 general election and won with 13,114 votes (92.3%).[5] He had no reelection challengers until 2010 when he beat Democrat Eric Wolfgang 72.8% – 27.2%.[6] Democratic candidate Sarah Speed faced Gillespie in 2012 and he kept his seat with 20,236 votes (60.7%) to her 13,127.[7] Gillespie had no ballot opposition in 2014 and 2016. Democrat Michael Wascovich received 35.6% of the vote in 2018 to Gillespie's 64.4%.[8] In 2020, he won against Democrat Fred Owens, 65.4% to 34.6%.[9]
In 2022, Gillespie lost in the Republican primary to challenger Joe D'Orsie, 40% to 59%.[10]