No no, you pay not the gain of transaction, you pay tax for the whole amount.
And then deduct the amount of fiat you used to buy crypto.
So, you buy 100 USDC for $100 - you report that in your tax information (for use in following years tax deduction).
Then some time ago, you sell 110 USDC for $120, you pay tax for the amount $120 - $100 (unless the $100 was used year earlier, then you can use just $50 as a cost).
If you didn't report any costs (meaning buying crypto) you pay tax for the whole $120 amount.