Please please please do not perpetuate the myth that insulin makes you gain weight. It is an incredible dangerous thing the suggest - just look up diabulimia.
Everyone, whether diabetic or not, uses insulin. The only difference is that we inject it.
Excess insulin can cause weight gain but unless you have insulin resistance, that is not an issue.
5kg is not a low of weight. As
@lovinglife mentioned, a common symptom of Type 1 is weight loss. It is common to put that weight back on.