I see in Netanyahu's press conference this evening...asked if Israel told Trump about the recent attack on Iranian gas fields, Netanyahu says that "Israel acted alone".
Won't this have all been coordinated through a normal ATO process, which means the US will have had full awareness of the targets prior to the strike? Or is Israel doing their own thing some of the time?
Yes.
It means that Trump now owes him a favour, especially if he has a paper trail detailing the background briefing - and of course a copy of the ATO Frag.