My grandmother recently passed away and I am unaware of a will. My aunt from out of state came down to settle the estate. She and my father were my grandmother's only children and my father has been estranged/missing for over 30 years. My grandmother believed that someday he would return, so if she had a will, I know that he would not have been excluded. I believe that my aunt may have my father declared legally dead in order to claim the full inheritance. My question is:

As my father's only child, would I be entitled to his portion of my grandmother's inheritance should he be declared legally dead?

I am reading that the estate goes to the children. If no children survive, then grandchildren. That, of course, is not the case in your situation.

If you believe your aunt committed fraud you need to retain an Attorney.

If your father died AFTER your grandmother you would inherit from his estate, not hers - but that is also not the case here (or so it appears).

Grandchildren in Texas do not have the right to inherit when there are living children.

Are you POSITIVE there's no will?

I think you should speak to an attorney - especially because my reading of TX stautes for intestacty is different than JKT's. I found this:

Sec. 38. PERSONS WHO TAKE UPON INTESTACY.
(a) Intestate Leaving No Husband or Wife.
Where any person, having title to any estate, real, personal or mixed, shall die intestate, leaving no husband or wife, it shall descend and pass in parcenary to his kindred, male and female, in the following course:
1. To his children and their descendants.

So I think what happens in your case is 1/2 goes to the aunt and 1/2 goes to the children of your missing/dead father.