The “life of man on earth” is the GIFT, while the “life eternal of man in heaven” is the REWARD.
Life on earth is considered a GIFT because we did not have to earn it to have it; God gave it to us so freely and unconditionally without us ever having the opportunity to even ask for it.
On the other hand, life eternal in heaven is a REWARD because it is something we have to deserve in order to have. Only those who are doing the will of God may receive it. And by the grace of God, may we all receive it.
Nonetheless, for all the infinite beauty of the reward, may we all live for the GIFT, not the REWARD, lest we negate this most beautiful life on earth—“the greatest gift we have ever received from God.”