It's a 2000 engine new ported pump and O-ring...just had it down to put the new pump and timing set in it.

 

It's pump or pump related hardware then oring etc. oil pressure leaves the pump flows down galley drilled down the side of the block into oil filter out of oil filter and directly up to oil pressure sensor the out to valvetrain and mains from there. Oil filter maybe collapsed? But if your mechanical gauge shows low pressure something major with pump happened.

Pull it apart and check bypass maybe but I wouldn't mess with a pump having problems put a new one in it and hope it didn't hurt bearings. It wouldn't be first time I've seen a LS turn cam bearing with same end result also.