Is the gtt the only method for determining hypoglycemia?

No. A full glucose tolerance test is not necessary to determine hypoglycemia that occurs as an isolated incident that involves reduced food intake. For the most common type, reactive hypoglycemia, a glucose tolerance test is the best way to make the diagnosis.
No. It depends if it occurs after eating or fasting. Most GTT tests do not really evaluate "hypoglycemia" go to a real doctor with your symptoms.