That the phrase "you guys" is genderless, being a plural "you" that Standard English has lacked since the demise of "thou," is not disputed. The word "guys" itself, perhaps by extension from that phrase, seems to be becoming genderless for many English-speakers, too. One forum thread at Antimoon (a site for ESL people) comments about the usage. American Heritage Dictionary, Merriam-Webster, and Wiktionary agree.
And in the wild:
Even though the majority of you guys [in the apartment] are male...
The above was Forrest, last night. Below is my ethnic studies teacher, today:
And Lucia Gonzales was a woman [in the workers' movement]. Most of these guys were men.
Language change is interesting. :)