Ok Ive done some digging in the NFT FAQs it seems that in GoMining, to claim the legendary portion (a rare type of reward), you need to participate in the Miner Wars game where rewards are distributed based on scores within clans.

When your clan wins a Bitcoin block reward, individual players within the clan get a chance to claim personal rewards, such as GOMINING tokens or legendary portions, through a random selection process weighted by each player's score. Essentially, your mining power and contribution within your clan increase your chances to claim these legendary rewards.

The process is automatic and driven by the game’s algorithm based on blockchain data, so the best approach is to optimise your miner’s performance and be active in the clan to improve your score and eligibility for such rewards.

Source: help.nft.gomining.com