From 2001 until 2013 Gremmels was a member of the municipal council of his hometown Niestetal and, among other roles, deputy chairman of the local SPD parliamentary group. Since 2001, Gremmels has been a part of Kassel's county council (Landkreis Kassel). In November 2016, Gremmels was nominated as SPD candidate for the Kassel constituency. He was elected to the Bundestag in the 2017 general election where he secured 35.5% of the first vote in his constituency. Thus he moved to the 19th Bundestag as a directly elected parliamentarian. In parliament, Gremmels serves on the Committee on Economic Affairs and Energy and on the Committee on Petitions.