How are credit card awards in Bitcoin taxed?

For example, Gemini is offering a credit card that pays 3% or 2% or 1% rewards for each purchase when each credit card transaction posts.

https://www.gemini.com/credit-card

I am assuming the rewards are taxed, but not sure how. I see a couple of possibilities:

1. INCOME. Could they be earnings income, which would be subject to income tax?

2. CAPITAL GAINS. When I eventually sell the Bitcoin would it be capital gains(loss) and if so would it then use a $0.00 basis amount?

The credit card offer seemed like a great idea, but on reflecting I cannot figure the tax implications. Anyone know more about this?

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