If Ian's played simpy/clingy, Gillian is disinterested, cuz people who play cuck Ian are into the girl getting the salad, the cake and the chef's dick (aka cheat on him, dump him and get him back easy).
If you play a Chad, Gillian shows eagerness into getting him back which plays into what Chad Ian players would want (being the reward after getting played before).
That's not the case anymore. In the current version of the game Gillian mentions wanting to go for a coffee together and acts just as interested also when Ian is being open about still having feelings for her, instead of only when he brushes her off.
(regarding different tone, i think it's very much up to interpretation if Gillian is being "disinterested" or feeling more guilty and thus unsure if she should reach out towards Ian who told her their breakup affected him to the point he still isn't over it, vs one who is like "eh, whatever and water under the bridge" at this point)
The tone and expressions Gillian takes are different. Her expression even lights the fuck up when Chad Ian agrees.
Gillian's expression after the coffee bit is actually literal opposite of "lights the fuck up":
Python:
if v11_gillian_talk == 4:
//...
i "Well, I feel like I already said my part. And I think you said yours."
g "Yes..."
i "I should get going, then. Perry and the others are waiting for me at the bar."
$ fgillian = "n"
g "Of course... Say hi to them for me."
g "And, Ian..."
g "If at any point you feel that enough time has passed, and you feel like it..."
g "I wish we could have a coffee or something like that, so I can keep knowing what's up with you and how things are going for you."
i "I'll keep it in mind. Goodnight, Gillian."
$ fgillian = "sad"
She goes from neutral to
sad. She has no such reaction on other paths.
In their second interaction about it, in Ch.13, if Ian has acted cold Gillian starts her request with sad expression, and if Ian agrees she goes to neutral.
Python:
if gillian_stop:
//...
"Don't let me keep you."
//...
$ fgillian = "sad"
//...
g "Um..."
g "Hey, I know you'll probably say no, but... Would you like to grab a coffee one of these days?"
$ fian = "sad"
menu:
"When?":
//...
$ fian = "n"
i "Coffee? Sure... When?"
$ fgillian = "n"
Which i guess does technically count as relative lighting up, but "neutral" is not ecstatic by any means and is actually the exact same expression she keeps throughout when interacting with more friendly Ian.