Do we know Philip's back story? When and why did he and his first wife get divorced?
I thought that round the time he and Kirsty first got together after her miscarriage, or possibly when he was first introducing Gavin and Kirsty to each other, he said something about having split with Gavin's mother when Gav was about 6. I could easily have misremembered, though.
That would make it about 25-30 years ago, which would mean Gavin's comments about having to tell his mother about Philip remarrying were even more weird, because if they split that long ago, why would Philip be expected to have that close a relationship with her? It must be at least a decade or 15 years since Gavin turned 18, so they wouldn't have had to talk about school or anything which would need them to be in regular touch. So I am not sure if it is that long, nor how old Gavin is, but I assume somewhere around 30, because he's closer to Kirsty's age than Philip is.
Kirsty could easily say to Philip that Gavin had popped in unannounced, and please can he ask him to give the keys back just ring first, in case it's inconvenient. She doesn't have to mention (at this point) that Gavin is a complete nutter who picked an argument and was probably snooping and thinking no one was in.
I am not sue if he is a partner or employee. I don't see why he should have any more right to knowing about Philip's personal finances than Kirsty does; less so, as I think she has a stake in the house, if not equal. If he's a partner, then I would expect him to know how the business is doing, but I suspect snooping around on Philip's laptop has given him more info than he had a right to know, and I suspect it's not a new thing, either.
If Kells breaks up with him, I predict him moving into the spare room with Philip and Kirsty.