is it possible to gain a lot of weight when on chemo?

Yes, there are a number of reasons that many patients gain weight while on chemotherapy nowadays.

Edema (fluid retention), steroids used as part of the regimen, better antiemetics available today (like Zofran) that eliminate nausea so patients can eat and they often do, especially comfort foods high in fat and calories.

That said, looking at a number of recent pictures, I don't really see a weight gain, but rather a not very healthy looking Hugo. I don't think he's faking anything, and things are not well with him. Huffingtonpost uses a lot of recent Getty photos and many show him (IMO) to be less "puffy" than he was before all this and with poor skin coloration.