during my holidays i put my car approx 2 times a week under a GSM tower and did never ever notice any influence on gps reception.
I dont have the BU GPS and hereover in europe ist 900/ 1800 MHz ( usa 1900 ) but i think it doesnt change much the 100 MHz theorically difference.
Also GArmin builds GSM phones with GPS so i doubt on interference in general but we never now - perhaps bu GPS is a bit sensible
felix