To adjust GPM on a quote, you have a few options
1. Adjust it globally on the job. To do a global adjust, you can type into the GPM ribbon at the top of the job's screen. When typing a value, it will prompt a window to see how/where you'd like to adjust the values from. "Advanced Options" will be the default which allows you to toggle Materials, Labor and Charge items to factor the GPM value from. Selecting "Even" will factor the GPM adjustment throughout all categories. Lastly "Category Adjust" allows you set a dollar value for each of the three sections within a job.
2. Adjust it per item. In addition to a global change, you can also set GPM per line item. This can be done on Materials, Labor and Charge items but locating the "GPM" column and inserting a value there