There is no HMRC guidance on NFT’s or other cryptoasset rewards earned whilst gaming, so you would need to examine the specific circumstances to determine the tax treatment.
It is likely that such ‘in game’ earnings will be taxable as miscellaneous income, subject to income tax. The sterling value of the NFT or cryptoassets at the date of receipt will be the value of the taxable miscellaneous income.
This same value is the capital gains tax acquisition cost of the NFT or tokens; to be deducted from the proceeds of a later disposal of the tokens (which is subject to capital gains tax).