> As a non-native speaker, isn't "guys" a correct term regardless of the gender?

"Guys" can be used in both a male-gender-specific sense (for which the female-gender-specific rough equivalent is "gals"), or in a gender-neutral sense; there is no structural way to distinguish the uses. Like most places in English where a word has both a male-gender-specific use and a gender-neutral use, the latter is somewhat controversial with certain groups, who prefer unambiguously gender-neutral terms rather than dual-use terms that can either be male-gender-specific and gender-neutral.