The usual advice is for someone with Type 1 is anything up to 10g of carbs can be eaten between meals without insulin.
Alternatively, you could take insulin for the snack.
My approach is, if I feel hungry, I eat what I fancy and inject appropriately.
Depending upon how I feel, this could be nuts or fruit or crisps or muesli bar or chocolate.
I rarely snack on cheese because, unless you have those individually wrapped slices, it is a pest to carry when I am out. And I am not happy with the excess wrapping on some of what I eat as it is.
Tomatoes and avocado are not great for a similar reason.
I don't focus on low GI but chocolate is low GI due to the fat. And dark chocolate is low carb (if that bothers you).