Quick answer: DGB, from low to high.
And these are the same strings (4th, 3rd and 2nd) as in any guitar, tuned in the same way as in any guitar, and that is why playing on a Loog is the same as playing on any guitar, only simpler and with less things to remember.
Now, the Loog Guitar is all about exploration, so you can also use other string-configurations and tuning if you want to: Open A, Open G… whatever works for you!
(You might also want to check "Can I play ANY song on a Loog Guitar?" below)