My first thought would be nuts. They're not very low in carbs but I would think they're low enough to be fine for most people. For me personally a few handfuls of nuts does nothing to my blood glucose levels. They're packed with calories and nutrients and good snack food, so you could maybe eat nuts after meals or as a snack between meals. Nuts aren't at all cheap if you buy them in small bags from a supermarket but there are suppliers who will ship nuts in large quantities for a more reasonable price, compared to buying them in small bags from a shop. If you Google 'Nuts in bulk' you'll find a couple of suppliers.
Another option is cheese - also packed with calories. Eating cheese and nuts together would be a meal in itself and would very probably help you gain weight I'd imagine.