Am I the only one who really really doesn’t even want to see Chell in Portal 2? Anyway I hope valve is smart enough not to do it. My guess is that that Gabe comment has been redacted. Valve has really emphasized that though they take place in the same universe and slightly overlap, Portal and Half-Life are completely separate games and will most likely remain that way. Next, Chell is one of Half-Life’s classic mute characters. If we’re going to see her in Ep3, she would have to talk which would be weird after all her mute adventures. Or she could be a playable character in which case the next problem applies more. Then, it’d be weird for her to use anything other than a Portal gun if she were to take part in action. She lost the portal gun so she can’t use it and it’s not very practical in the outside world. And her with an smg just doesn’t fit. Next, it’s hard to think of a plausible way for them to meet.
TL;DR it’s not gonna happen and it’s not a good idea.