Xbox gold card trying to give me game pass

That means you currently have a subscription to Ultimate. You can cancel Ultimate or wait for it to run out and then add the gold card in order to get just the 3 months of Gold. You can cancel at http://account.microsoft.com

so from my understanding If I got purchased 3 years of xbox live gold,say even if I purchased these via 3rd party online seller which sells xbox gold cards and then redeemed these cards via my xbox one giving me years of gold then went and subbed to xbox
game pass ultimate for the 31 offer it'll give me 3 years of game pass ultimate subscribtion?.If I remove my card details afterwards I sub to game pass it wont lose the game pass after a month and go back down to regular xbox live gold membership for the rest
of the 3 years?..

If it's for Xbox, then Game Pass, Xbox Live Gold, Xbox Gift Card, or give a digital game as a gift.

It now depends what you actually bought and what type of subscription you have.

You say you have the Game Pass Ultimate subscription, but above you said you have bought a 1 year Xbox Live Code.

Since Game Pass Ultimate is more expensive, a "normal" Xbox Live Gold card won't give you the full year for your Game Pass Ultimate subscription.

For a 1 year Xbox Live Gold prepaid card you "only" get 4 months of Game Pass Ultimate.

See here: https://www.xbox.com/live/gold/upgrade under the "Frequently Asked Questions" and open the question "After I join Xbox Game Pass Ultimate, can I still redeem
Xbox Live Gold and Xbox Game Pass codes?" to see which code gives you what amount of Game Pass Ultimate.

If the Credit should be for Xbox Live Gold or Game Pass subscriptions I would recommend a prepaid Card for that. If you want to give him credit for buying games or dlc you could add the prepaid Xbox credit cards for it.