there are 2 entrances directly going to the GB5 parking, one is mentioned above,
the other is along dela rosa st.
if you're coming from Edsa, take Ayala Ave.
the 3rd traffic light would have a makati ave. sign. turn left. (you're now along makati ave.)
immediately on the first corner(ayala museum) turn right. (you should be along dela rosa)
you should see a parking sign to your left. (under the footbridge).

if you missed dela rosa and went pass ayala museum, take the GB parking right after GB4, go straight when you're down there and it would still take you to GB5 parking.