Gnesios (γνήσιος) means genuinely born, legitimate, or authentic — describing something or someone that is the real thing rather than a counterfeit. It appears 4 times in the New Testament. Paul uses it to describe Timothy as his 'true [gnesios] son in the faith' (1 Timothy 1:2) and the 'loyal yokefellow' (Philippians 4:3) as gnesie (genuine companion).
Gnesios belongs to the language of authentic relationship and genuine character. Paul's use of it for Timothy is deeply personal — he was not Paul's physical son but his gnesios son in the faith, meaning their relationship had the weight and legitimacy of a true father-son bond formed through the gospel. In 2 Corinthians 8:8, Paul tests the 'genuineness' of the Corinthians' love by pointing to the generosity of the Macedonians. Authenticity in faith is proven by action, not just claim. A gnesios love gives. A gnesios relationship serves. The genuine Christian life cannot be faked indefinitely.