Their website says
"Two row malt, shredded fresh Yellow and Blue ginger, Cascade hops, Jamaican cane sugar, Royal Jamaican new crop rum."
Alcohol is 4.4%
So it's a beer with ginger (and rum, I'm guessing white), not what we'd traditionally call ginger beer
I'd start with a nice easy drinking ale just using Ale Malt, Sugar and Cascade
So possibly something like
3kg Ale Malt
350g Raw Sugar
Cascade to maybe around 20 IBU (maybe, can't find a figure anywhere)
LOTS of ginger, if it's got a big bite (and unless you're using chillies) you'll need quite a fair amount, be prepared to pay a fair bit, I'll say at least 1.5kg if not 2
Then enough rum (probably appleton estate would work well) to bring it up from 4% to 4.4%
Depending on how much of the rum shines through (I've never tried this), you may want to back off on the sugar/ malt and use more rum, or the other way around