No-Fee United Miles Earning
This is the no-annual-fee way to earn United miles on everyday spending. You get 2x on United purchases, gas, and local transit/commuting, plus a small in-flight discount. There's no priority boarding and no lounge days, and bags aren't free out of the gate—you have to spend $10,000 in a calendar year to unlock two checked bags. Treat it as an earn card first, a perks card second.
If you mainly want free bags and better travel bennies without a spend hurdle, look at United Explorer or higher. If you just want to rack up miles without paying a fee (or need a United keeper card), Gateway is painless.