The in-demand Dutchman was on the radar of Manchester United for several months, and the Red Devils were seemingly the frontrunners for a January deal after being knocked back by PSV Eindhoven in the summer.
However, Liverpool unexpectedly stole in to hijack Gakpo's move to Old Trafford, and they agreed a £37m deal with PSV, rising to £44m with performance-related add-ons.
Gakpo is without a goal or assist in his first six appearances for Liverpool in all competitions, during which time the Reds have won just once against Wolverhampton Wanderers in the FA Cup.
Speaking on a YouTube show hosted by former Netherlands international Andy van der Meyde, Koeman compared Gakpo's situation to that of Ryan Gravenberch, who has also failed to set the world alight since joining Bayern Munich from Ajax.
"The level in England is higher than in the Netherlands, but they are also young boys, aren't they? Like Gravenberch going to Bayern and not playing. Then that is difficult," Koeman said.
"You can see that he has ended up in a team that is not doing well. Then it becomes more difficult for him, as a new signing. You are tested right away.