Does the discharge of white fluid before urination necessitate ghusl?

It appears that what you have described is "Wadi" and not "Mani." Wadi is a thick, white fluid that exits after urination, whereas Mani is a thick, spurting fluid that exits at the peak of sexual arousal. Wadi is impure and nullifies ablution (Wudu), but it does not necessitate a ritual bath (Ghusl). Rather, it requires performing Istinja (washing the private parts), Wudu, and washing whatever it has stained of one's clothes or body.