The got was they already had. The US has been able to do pretty much everything the wanted in Greenland since the 2nd world war.

Its a very good deal for Denmark too, especially on the long term with a more sane person than Trump in charge. The benefit are good for both parties and gives Denmark a role of exclusive ally to US in the region, something like South Korea in the East Asia, before Trump broke with them too.

But at a steep price.