I lost 6 kilos in weight after my resection and during chemo and was advised by a dietician to pile on the calories, so full fat milk replaced semi skimmed, Greek yoghurt replaced natural and more bananas and eggs than usual. The result is stabilisation of weight but no significant gain yet. But having finished chemo and now building muscles again on a bike, and also currently skiing, I hope that this will result in weight gain.
Good luck with your diet!