Your result
Possible
There may well be a claim here, and the person who should be looking at it first is not you - it is your parent.
Thailand has no ancestry route and no grandparent rule. Section 7(1) of the Nationality Act passes nationality from parent to child, in or outside Thailand, with no generational limit and no rule that a citizen by descent cannot pass it on. So a Thai grandparent reaches you only through your parent. If your parent is Thai, you are Thai. If your parent is not, the line stopped before you.
The point that is easy to miss: your parent may already be Thai without ever having held a Thai document. Thai nationality by birth is automatic. It is acquired at birth by operation of law, not granted by registration, and the birth certificate, house registration and ID card are evidence of it rather than the source of it. So a parent born to a Thai parent is very likely Thai now, unregistered, and unaware of it - and there is no age limit or deadline on establishing that. People do it in their fifties and sixties.
That makes your parent the right applicant to look at first, and it is a better use of effort than anything you can do on your own account, because your position follows from theirs.
The things that could break the chain, so you know what you are testing. If the Thai link on that side was your grandfather and he was not married to your grandmother, legitimation was needed and may never have happened. If it was your grandmother and your parent was born abroad before 1992, note that the old father-only rule was made retrospective by section 10 of the Nationality Act (No.2) B.E. 2535, so that is usually a fixable objection rather than a fatal one. And if your grandparent had already lost or renounced Thai nationality before your parent was born, there was nothing to pass on.
What happens next: gather your grandparent's Thai records - birth certificate, ID card, house registration - and take them, with your parent, to the Thai embassy or consulate for the country your parent lives in. Establish your parent's position first. If your parent has died, the same evidence can still be used to show the line, but expect it to be harder, and get advice before committing to it.