3 added caveat about use of guys.
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From the perspective of a woman of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

In my cultural subgroup, we would not call a group of women gals, folks is just too folksy, but guys is OK. Among the women I play tennis with, we sometimes call ourselves guys and sometimes ladies, but we never call ourselves gals. A woman would refer to her family, but never to her folks.

Conclusion: Use guys, but if a woman in the group objects, apologize and don't do it again with some thought for your audiencethat group. Never use gals. Even within the US, there are wide differences in language that depend on age, geography, and degree of education education.

From the perspective of a woman of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

In my cultural subgroup, we would not call a group of women gals, folks is just too folksy, but guys is OK. Among the women I play tennis with, we sometimes call ourselves guys and sometimes ladies, but we never call ourselves gals. A woman would refer to her family, but never to her folks.

Conclusion: Use guys with some thought for your audience. Never use gals. Even within the US, there are wide differences in language that depend on age, geography, and degree of education.

From the perspective of a woman of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

In my cultural subgroup, we would not call a group of women gals, folks is just too folksy, but guys is OK. Among the women I play tennis with, we sometimes call ourselves guys and sometimes ladies, but we never call ourselves gals. A woman would refer to her family, but never to her folks.

Conclusion: Use guys, but if a woman in the group objects, apologize and don't do it again with that group. Never use gals. Even within the US, there are wide differences in language that depend on age, geography, and education.

2 rewrote 3rd paragraph. Added conclusion as 4th paragraph.
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From the perspective of someonea woman of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

I think it is iffier toIn my cultural subgroup, we would not call a group of women gals than to, folks is just too folksy, but guys is OK. Among the women I play tennis with, we sometimes call a mixed groupourselves guys and sometimes ladies, but we never call ourselves gals.   A woman would refer to Galsher family, but never to me means women of beyond a certain age who sit in the rear of the car while the menfolkher folks sit up front.  

With the last sentence, the reader has decided I am a snob. InConclusion: Use guys with some things I amthought for your audience. We never, or very rarely Never use gals. Even within the US, mention class either here orthere are wide differences in ELUlanguage that depend on age, but why class is a dirty subject is another questiongeography, and degree of education. 

From the perspective of someone of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

I think it is iffier to call a group of women gals than to call a mixed group guys.  Gals to me means women of beyond a certain age who sit in the rear of the car while the menfolk sit up front.  

With the last sentence, the reader has decided I am a snob. In some things I am. We never, or very rarely, mention class either here or in ELU, but why class is a dirty subject is another question.

From the perspective of a woman of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

In my cultural subgroup, we would not call a group of women gals, folks is just too folksy, but guys is OK. Among the women I play tennis with, we sometimes call ourselves guys and sometimes ladies, but we never call ourselves gals. A woman would refer to her family, but never to her folks.

Conclusion: Use guys with some thought for your audience. Never use gals. Even within the US, there are wide differences in language that depend on age, geography, and degree of education. 

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From the perspective of someone of retirement age, yes it is OK to call a mixed group "guys". (Further cultural ID: Boswash Corridor, US, middle class, associates all highly educated.)

Furthermore, from travelling in the western US, I can attest that if I, as a woman in a mixed group, had objected to being addressed as "you guys" by a waiter I would have starved to death.

I think it is iffier to call a group of women gals than to call a mixed group guys. Gals to me means women of beyond a certain age who sit in the rear of the car while the menfolk sit up front.

With the last sentence, the reader has decided I am a snob. In some things I am. We never, or very rarely, mention class either here or in ELU, but why class is a dirty subject is another question.