The galley gaskets are internal gaskets and if faulty the oil pressure will suffer.
Replacing the gaskets rectifies this issue.
Is that not the reason why you replaced the galley gaskets ?
The oil pressure gauge may be faulty, however more chance that it is the oil pressure sender switch.
Get the oil pressure checked with a stand alone gauge.
And remember that you do not have a problem until you have a problem.
PS: Nissans "own" 5W30 is low quality oil.