First off, I would point out my opinion that I think girls can be very good gamers and have known my share who were quite capable.
However, I think that its over simplifying for everyone to say that gender makes no difference or has no correlation just because its not something like a strength contest. Beyond just mass and appearance there are a lot of physiological differences between the sexes that can play into things. As a general rule its been observed that men seem to see things better in a 3d space and keep a more accurate picture of rapid movement which gives them a small advantage in 'twitch' type gameplay. On the opposite side of that, women are generally more detail oriented(both noticing and remembering) and are better at following steps in a linear progression, so overall they have advantages in puzzle and adventure type games. If you want to think about it in terms of generational evolution, once upon a time often men needed quick responses for hunting while women needed to be observant for gathering and be skilled at piecing things together in the order needed for the many other tasks they would perform(everything from crafting a basket to cooking).
Thats just physiology too, it leaves out social norms. Culturally it was acceptable for guys to be gamers far earlier than girls(even now, really) and while the number of girl gamers is blossoming most are being directed to the casual game market. I'm not trying to start the whole 'casual games are not games' argument, but the skills you pick up in them are -very- different than the rest of the market and likely don't lead to a lot of overlap like there are between other genres. If you played counterstrike, obviously you could probably pick up team fortress without too many problems. I would wager though that in a comparison of a new gamer whose only experience was WoW vs one whose only experience was Farmville, the WoW player would seem much more skilled quite quickly because they would already be accustomed to ideas like WASD, splitting attention between keyboard and mouse and using hotkeys and could focus on gaining talent in the game rather than what most players see as basics. This is something that will change as our social norms expand to encourage girls to try out different genres instead of just saying that shooters are for boys, but for now girls who have had the chance to really sink their teeth into more intense gaming without societal pressure from family or friends that they should be doing something more suited to their gender are too few and far between.
Phew, that was a long rant and obviously full of generalizations. I figure my short answer would have sufficed for the OP, but I did want to point out that its not always so simple as 'yes they can do it.'