The site here is meant only to show that Bull was killed in Brussels.  He was assassinated at the door of his apartment but the precise location needs to be found.

Gerald Bull

Gerald Vincent Bull (March 9, 1928 – March 22, 1990) was a Canadian engineer who developed long-range artillery. He moved from project to project in his quest to economically launch a satellite using a huge artillery piece, to which end he designed the Project Babylon "supergun" for the Iraqi government. Bull was assassinated outside his apartment in Brussels, Belgium.^{[2][3][4][5][6]}

• http://en.wikipedia.org/wiki/Gerald Bull