Ah, that changes things a little bit then.
Realistically, any gaming laptop will have limited battery life as the majority of the components will be running in some way at all/most times.
Something you can game on at any decent level will struggle to meet 8+ hours battery life for work purposes - it's built for performance, not battery life.
Given your budget, take a look at Sager Notebooks. I'm trying to justify buying one myself at the moment. Pricewise it seems to be the best "bang for your buck" and, with the money left over, grab a couple of extract battery packs & you'd be sorted!