Go to Gimp "Edit" (should be on the top right) then select "Preferences" (near the bottom of the Edit menu) The preferences menu will pop up then click "Tool options". (should be on the left of the window)
Next go to "Paint options shared between tools"... Deselect or uncheck the boxes or tools (Brush, Dynamics, Pattern and Gradient)
Hit okay and it should stop doing that... Unless you reinstall gimp or it crashes before saving the option and closing out of the program.