I never experienced any sort of weight gain...Yes, I was mostly immobile just lying in beds or chairs, but I did not eat much at all.
You will mostly lose your sense of taste unfortunately, so texture (for me) becomes more important. Like others will say, it mostly depends on what you can and will eat. Try not to focus too much on potential weight gain or anything, if you can eat you will need to.
Pretty much the only thing I found that I was able to remotely taste was ketchup - so I ate anything I could put ketchup on: french fries, chicken nuggets, tater tots, etc.
But still, I lost about 20 pounds and was underweight when I finished chemotherapy.
I/O - Jan 3rd 2011 (Diagnosis) 95% embryonal, 5% mixed (trace teratoma)
CT Scan - April 2011 (Relapse) 1.8cm enlarged node
RPLND - May 10th 2011, 1 node positive for embryonal carcinoma
2x BEP - Began June 20th 2011, ended July 25th 2011
CT Scan - November 17th 2011 (Clear), February (Clear), May (Clear)