It is Time for Truth – I know many who do, but I am not so comfortable discussing it. If it is okay for girls to learn Gemara, it is only with several conditions attached, and one of those is that it should be done b’tzina. In other words, it shouldn’t be a topic of conversation for the internet, imho.
But just to respond to your question, most girls can’t learn Gemara in depth, but that is not because they are not capable of it – it is because for hashkafic reasons, it is not considered an important thing for girls to learn, so even in those schools in which Gemara is taught, it is usually only taught as an optional side subject for the year of seminary. It is not considered one of the main courses, and it is not the subject that they are encouraged to continue learning after seminary.
Those girls/women who do consider Gemara learning to be important (whether rightly or wrongly) do continue learning Gemara in depth for several years and are quite capable of doing so. They can probably learn better than the average man because they are not the average women.
However, most girls/women feel that a girl’s time is better spent in other ways, so they do not spend any or much time on Gemara which is why they can’t learn it. But they may excel at Tanach or some other area of learning that they do consider to be more important.