Two ways you can do it.

If you ONLY care about access to the games + Gold, you can open a new account every month to take advantage of the “First Month for $1”. With that, you can have GPU for $12 per year, essentially free.

If you’re too concerned about bankrupting Microsoft, the other alternative (that a ton of people me included) do, is stacking 2 - 3 years of GOLD membership ($60 per year). Claim it all, then buy a month of Game Pass Ultimate ($15) and watch it become 3 years of paid Game Pass Ultimate. By doing this, you basically got the upgrade for “free” which is what most people mean when they say “Enjoying GamePass Ultimate for free”, because they already intended to pay for Gold.