Learn Turkish: Bu nitelikleri kazanmış olan gençler çok şanslı. - The young people who have gained these qualifications are very lucky.

kazanan uses the standard -(y)An subject relative clause, which usually gives a present or general meaning ('the young people who gain'). By using -mIş olan, you specifically emphasize that the action was completed in the past: 'who have gained'. It frames the gaining as a finished prerequisite for them being lucky right now.

Could I just say kazanmış gençler without using olan?

Yes, you can use the -mIş suffix directly as an adjective to mean 'the having-gained youth'. However, adding olan (from olmak, meaning 'to be') clearly builds a 'who have [verb]ed' structure. This is heavily preferred in Turkish for clarity, especially when the relative clause contains its own objects like Bu nitelikleri.

Why does nitelikleri have the accusative -i ending if the phrase acts as the subject of the main sentence?

Because Bu nitelikleri (these qualifications) is the specific direct object of the verb kazanmak (to gain) inside the relative clause. Even though the entire block Bu nitelikleri kazanmış olan gençler is the subject of the main sentence, the verb inside the relative clause still demands the correct case markers for its own objects.

Does nitelik mean the same thing as özellik?