With T1D it only makes sense to take these post-workout. Most people take them post-workout anyways because that's the purpose, to help rebuild the muscle when you're done stressing it. But with T1D it especially makes sense to take it after because you need to heavily dose for a mass gainer and you don't want to risk overdosing and then having a nasty hypo that will cancel the workout. So take it after.
However, I will add that the bigger concern is actually underdosing for these monsters. I hope you are carb counting because if you don't get it right, these things will send your glucose out the roof, and I mean to the moon. You will spend the rest of the day trying to bring it down. Avoid this by carb counting. Even if you don't want to regularly carb count, figure out your ratios anyways and apply it every time you use a mass gainer.
