I have a sore on my gum at the top what is it?

Unknown. Such sores can be triggered by viruses, bacteria, trauma or allergic reaction to ingested material. Your description is too vague to narrow the cause.

Size, color, exact location, duration, raised/flat/cavitated, painless/sore, history of trauma/burn, drainage, etc. Is it viral, bacterial, fungal? Is there the possibility of an infected tooth? Is it benign or not. This is a general information site, not a diagnosis site. Your best option would be to see a Dentist for an examination.

You really need to see a dentist to have that lesion evaluated. "Sore on the gum" is not descriptive enough.