How do GIUK defences constrain NATO as well as Russia?

If NATO controls the land and air bases around the gap, does the barrier only constrain forces coming from the north?

No. The outer sensor barrier protects NATO, but its gates and defended approaches also funnel Allied forces moving north and make some inward routes more predictable.

The comparison is Julius Caesar building one defensive line around a besieged city and another facing outward. SOSUS and the GIUK positions form the outer line that warns of attacks from the north. The same geography has limited passages and operational gates for NATO forces advancing inward. The barrier protects Allied sea lanes while narrowing and advertising some directions of Allied attack.