Barcelona managed to sign Gundogan because Gundogan is also on a free transfer and also, even though Mbappe has been linked with Real Madrid for a long time it is also not a guarantee that Mbappe will go to Real Madrid. So yes, anything can still happen and "for example" Mbappe to Barcelona, it is also still possible to happen. But unfortunately, Barcelona do not have stable finances in this moment and of course, with financial management that is not good then it makes Barcelona at this moment do not have big plans to bring in the most expensive players.
Barcelona are in heavy pressure considering recruitment of new players unless they are free agents. They have right now 26 players in the team, half of them foreigners. They got Gundogan and Martinez for free and collected few millions from Griezmann and Trincao. This money will not help them get any top player, they lack a real replacement for Lewandowski in case he get injured unless Xavi change his tactics.. For Mbappe, he will join Real Madrid, that's sure.