Metal Head Gasket failure

[Negative Creep]

Went out and bought a Nissan 200SX S13 at the weekend which had a recent engine rebuild. I looked through all the photos and receipts of it being done and saw it had a complete headset along with a Cometic 1.9mm metal head gasket. We checked the car over and it seemed fine, indeed drove home without missing a beat. Checked it yesterday and saw oil in the header tank. I was hoping this was just sludge in the radiator but I've just had a phone call from the garage saying the head gasket has gone. Seeing as it was only replaced 1500 or so miles ago and it is a strengthened item I am a bit confused as to why this has happened. I haven't seen the car yet or spoken to them so all I'm going on is what I've been told. Could it have something to do with the car only having done very limited miles in the last year? Or if the head wasn't skimmed when the gasket was replaced could that have caused it? Or is there any other way oil could get in the cooling system? Any ideas appreciated

[Negative Creep]

Turns out the last owner fitted a block from another car but didn't skim it, so I'm guessing that's the cause. Will I need another gasket as well or can it be reused (seeing as they're £100 a time)?

[Negative Creep]

Just to update this, I've found out the head bolts weren't correctly torqued, so that is seemingly the cause of the subsequent failure. I'm getting the head skimmed and pressure tested Monday then refitting using a new gasket, just to be sure. I'll be using a standard composite one as several people I've spoken to have said there's no need for a metal gasket when I won't be running more than 1 bar of boost