First
, Paul described both justification (Rom 3:24, 5:15,16,17) and eternal life (Rom 6:23) as gifts of God. Then he wrote Rom 11:29 - the gifts and calling of God are irrevocable. Since he had already described what he meant by "gifts of God", there was no reason for him to specifically list what he meant by "gifts of God".
3:24 - being justified as a gift by His grace through the redemption which is in Christ Jesus
6:23 - For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.
11:29 - for the gifts and the calling of God are irrevocable.
You left out the word "For" from Paul's statement. Romans 11:29 and you ignore "in Christ Jesus" from his statement Romans 6:23 It makes a difference if you leave out words. For instance, Romans 11:29 is expanding on Romans 11:25-28 In fact he says they will be saved as a consequence of or because of the fact that they are Israelites, and so they are beloved for the sake of their forefathers, again, as a consequence of the covenant God made with Jacob. So saying the gifts and the call of God are irrevocable is just saying God keeps his promises.
And eternal life is inherent in Christ Jesus. Romans 6:23 says the free gift of God is eternal life 'in Christ Jesus'. And Jesus said, if a man doesn't remain in him, he is cast forth as a branch and withers; and branches are gathered, thrown into the fire and burned. John 15:6