Gurjinder scored in the 11th and 20th minute, helping Lancers to their second win in nine games. He also scored the 100th goal of the 2015 edition.
Mumbai's Harmanpreet Singh converted a penalty corner in the 36th minute for a consolation goal.
Lancers are struggling with 16 points from nine games while Mumbai have 13 from eight games with only one win to show.
Mumbai travel to Lucknow for Sunday's game against Uttar Pradesh Wizards while Lancers head to Delhi to take on the Waveriders.