Not great, probably at the same level as Lopez recently ... that said, if Lopez doesn't play better than the current Donnaruma, then it 's only natural to want to play a young GK that can improve a lot as opposed to a 34 year old GK that has passed his peak and gets payed a lot more to play.
Unlike the hype here, I don't believe that Donnaruma is a great GK yet, and frankly was holding my breath when Lazio was shooting but did pretty well in two occasions, and messed up in one (and a half if we consider the empty jump when Alex got injured).
Personally I consider our GK the weakest point so far...