gender of Yours in latin...

In Latin, possessive pronouns, mine, yours, his, hers, theirs, etc have a gender ending which has to match the item or person referred to.

I think you are giving a watch to a male person, engraved with a message, telling him that you ( a woman) will always be his. So the possessive pronoun takes the feminine form.

Semper is the Latin word for "forever" or "always".

The Latin word for "yours" will either be "voster" (masculine) or "vostra" (feminine)

So

A woman saying "I am forever yours" to a man would use "semper vostra"

A man saying "I am forever yours" to a woman would use "semper voster".