Hi,
I am not aware of any method to do HDR exposure blending in GIMP. I am willing to stand corrected if anyone knows of a way to do it. I believe Hugin is a FOSS option to do what you want. I have not tested this myself. BTW, if you have a Canon camera you can use the serial number to legitimately download there editing software which does HDR blending as well as panorama stitching.