Is extreme weight gain a symptom of diabetes?

No: Extreme weight gain can cause type ii diabetes not the other way around.

Can be - get checked: When people develop type 2 diabetes, they don't respond normally to insulin, and their body makes more Insulin than usual trying to keep the sugar down. That increased Insulin can make people feel hungry soon after a meal and lead to overeating. Putting on weight worsens the Insulin resistance, and I do sometimes see people who have gained 50+ pounds just before learning they have diabetes.