Game pass 3 months or more cards

I don't know who told you that, but you can redeem game pass card up to 36 months for your account. And it is unlinked from gold.

I had 17 days left of game pass ultimate I decided to buy a 12 month Xbox live Gold card and a 3-month game pass ultimate card put in the 12 months first then the 3 months game pass ultimate card so that the 12 months would convert to game pass ultimate as well. Instead it only gave me 8 months game pass ultimate total as stated above.

When you buy more Game Pass Ultimate via on the store or with a gift card, it will extend the length of your Ultimate membership. It will inform you when it will expire as well. Another cool thing people don't often know is that if you have a 3-month regular
Game Pass redeemable code and you currently have an Ultimate membership, it will convert the 3-month Game Pass into 2 months of Ultimate membership.

There is no 12 month game pass subscription cards

You can buy GP time in blocks of 3 months, and store up to 3 years of subscription

To simplify, Gold was discontinued and became Game Pass Core. If he redeems the 3 month Gold card, he will have Game Pass Core.