It's a playful, ironic way to say "big kisses" or "lots of kisses". It's often used sarcastically or jokingly, especially when someone is trying to be affectionate but doesn't actually mean it.
Here are some examples of how it might be used:
* Friend: "I'm so glad you're feeling better!"
* You: "Thanks! Gross bisous!" (meaning, "I don't really want to kiss you, but I'm being polite.")
* Someone on the phone: "Okay, I'll talk to you later."
* You: "Okay, bye! Gross bisous!" (meaning, "I'm being silly, but I don't really want to talk to you anymore.")
It's important to note that "gross" in this context is not meant to be offensive or negative. It's simply a way to exaggerate the idea of "big" or "many".