Well, weight loss is definitely a side effect of undiagnosed T1, so the answer is that if you eat enough calories to gain weight you should now put it back on. You'll have insulin on board to convert all the food you eat so you'll be able to put weight on, whether you do or not will depend on how much you eat.
Sorry about the diagnosis, but you should start to feel much better now you've got the right one. And as you're now injecting insulin, you'll be able to have more carbs if you want them.