He's not at fault for the result, firstly let's be clear on that. But the 5 vs 2 and his performance as a whole is just a symptom of a much larger problem with him. Leaving aside for a minute his level/ability, stylistically he's just the absolute antithesis of a Liverpool forward.
He's not quick, he's not particularly strong (ridiculous really, given his size) and he plays with zero intensity. It means he spends far too much time on the absolute periphery of games not affecting things and then when the ball does come to him, he seems to consistently make poor decisions (perhaps because he's so scarcely involved in play). A lot was made of our improvement in form after he came into the side last season, but I think that had more to do with Jones and to a lesser extent Bajcetic than it did Gakpo - he had games like this last season too, where he was getting 9-10 touches a game and was just completely anonymous. One thing you would say is his pressing seemed much better last season, whereas currently he is not putting in anywhere close to the required effort on that front, which is part of the reason he's drawing so much ire right now in my opinion.
He's nominally our fifth choice forward yes, but he still plays a boatload of minutes due to injuries and the number of games we have (in fact he's played the third most minutes of our attackers this season), so his level needs to be better than it is. And it can be. But I still don't see him ever really properly fitting into a Liverpool side - he absolutely has his qualities and strengths, but they're not ones that are well suited to a side that looks to play with intensity in attack and defence, and whatever set-up/formation the new manager brings in, I don't see us moving away from that central identity.