Lifting below explanation from reddit:
There is currently an offer to subscribe to Gamepass Ultimate for €1. Gamepass Ultimate subs you to Gold, Gamepass and PC Gamepass in one subscription.
Subscribing to Ultimate will take the remaining time on your existing subs and add them together, giving you a longer sub for Ultimate (up to a maximum of 36 months) . For example, if you have 6 months Gold and 3 months Gamepass, you will end up with 9 months Ultimate. If you have 2 years of each, you will get 3 years of Ultimate.
For best value for money, extend your Gold sub (as it's cheaper than Gamepass), THEN take up the offer. Example, if you have 1 years Gold and 3 months Gamepass, you will get 15 months Ultimate. If you extend your Gold by another year, you will get 27 months Ultimate, essentially giving you the Gamepass for free (and saving a lot of money).
It also appears that you can now redeem a Gold only code after you have subbed to Ultimate. If you do this, it looks like the conversion rate is 3 months Gold = 1 month Ultimate.
This is all the games you will have access to on PC: https://imgur.com/a/Imm75V9
On the console version there are many many more.
Upgrade can be done here.