From the Ixalan set, we received double faced cards with lands on the back. I believe they count as all types on both sides while not on the field. Therefore:
You can't pick a double-faced card that is a land only on the back
Double-faced cards only count as their front face while not on the battlefield. From the rules on double-faced cards:
711.4a While a double-faced card is outside the game, in a zone other than the battlefield, or on the battlefield with its front face up, it has only the characteristics of its front face.
You cannot choose Gold-Forge Garrison as your land, as its front face is not a land.
708.4 In every zone except the stack, the characteristics of a split card are those of its two halves combined. This is a change from previous rules.
A double-faced card you chose would enter front face up
A double-faced card always enters with its front face up unless it enters “transformed”. If you choose a double-faced card that is a land on its front face, like Westvale Abbey, Golos would not put it onto the battlefield transformed, so you would get the front face.
711.8 A double-faced card enters the battlefield with its front face up by default. If a spell or ability puts it onto the battlefield “transformed,” it enters the battlefield with its back face up.