You can call them that, sure. Do you habitually call any specific man "boy"? Because, whether or not you intend it to be, many women find this specific pattern that you're exhibiting to be patronizing.
Maybe we should stop using the word 'girlfriend' and say 'womanfriend' instead?
And btw what about 'girls in tech' organization? Is it patronizing?
Boyfriend/girlfriend labels are equitable, and that relationship implies a familiarity for which a diminutive is typical and appropriate. Notably, this is not a professional relationship.
Girls in tech is about recruiting and supporting female children interested in tech. That's okay, because they're children. Your coworker is an adult, and it behooves you to treat them like one. For example, if she isn't acting like an adult, that's an issue to address in a professional manner, rather than call her a girl behind her back.
Ok i can swallow being accused of thoughtcrime (at least!), and will punish myself for it, but are you implying 'Girls in tech' is engaging in illegal employment of minors? That's a bold statement.