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.
That is why Barcelona are not in the race to sign top players at this moment, unless they are on a free transfer as I also said before. Also about Mbappe, the player is basically very likely to become a Real Madrid player later. But, as long as it has not been ratified then Mbappe still has a chance to be able to play for other clubs too. Because after all, the previous incident also Mbappe was very close to leaving for Real Madrid, but in fact PSG offered a new contract and the privileges given so that Mbappe failed to Real Madrid. With this case, it is very possible that if later there is a high and bigger fee from Real Madrid, then Mbappe could change course.