Objection 1. It seems that fortitude is not a gift. For the virtues differ from the gifts: and fortitude is a virtue. Therefore it should not be reckoned a gift.
Objection 2. Further, the acts of the gift remain in heaven, as stated above (I-II, 68, 6). But the act of fortitude does not remain in heaven: for Gregory says (Moral. i) that "fortitude encourages the fainthearted against hardships, which will be altogether absent from heaven." Therefore fortitude is not a gift.
Objection 3. Further, Augustine says (De Doctr. Christ. ii) that "it is a sign of fortitude to cut oneself adrift from all the deadly pleasures of the passing show." Now noisome pleasures and delights are the concern of temperance rather than of fortitude. Therefore it seems that fortitude is not the gift corresponding to the virtue of fortitude.
On the contrary, Fortitude is reckoned among the other gifts of the Holy Ghost (Isaiah 11:2).
I answer that, Fortitude denotes a certain firmness of mind, as stated above (123, 2; I-II, 61, 3): and this firmness of mind is required both in doing good and in enduring evil, especially with regard to goods or evils that are difficult. Now man, according to his proper and connatural mode, is able to have this firmness in both these respects, so as not to forsake the good on account of difficulties, whether in accomplishing an arduous work, or in enduring grievous evil. In this sense fortitude denotes a special or general virtue, as stated above (Question 123, Article 2).
Yet furthermore man's mind is moved by the Holy Ghost, in order that he may attain the end of each work begun, and avoid whatever perils may threaten. This surpasses human nature: for sometimes it is not in a man's power to attain the end of his work, or to avoid evils or dangers, since these may happen to overwhelm him in death. But the Holy Ghost works this in man, by bringing him to everlasting life, which is the end of all good deeds, and the release from all perils. A certain confidence of this is infused into the mind by the Holy Ghost Who expels any fear of the contrary. It is in this sense that fortitude is reckoned a gift of the Holy Ghost. For it has been stated above (I-II, 68, 1 and 2) that the gifts regard the motion of the mind by the Holy Ghost.
Reply to Objection 1. Fortitude, as a virtue, perfects the mind in the endurance of all perils whatever; but it does not go so far as to give confidence of overcoming all dangers: this belongs to the fortitude that is a gift of the Holy Ghost.
Reply to Objection 2. The gifts have not the same acts in heaven as on the way: for they exercise acts in connection with the enjoyment of the end. Hence the act of fortitude there is to enjoy full security from toil and evil.
Reply to Objection 3. The gift of fortitude regards the virtue of fortitude not only because it consists in enduring dangers, but also inasmuch as it consists in accomplishing any difficult work. Wherefore the gift of fortitude is directed by the gift of counsel, which seems to be concerned chiefly with the greater goods.
Source: Summa Theologica (New Advent)