What's the difference between "Gentle" and "Gently"?

Dylan Thomas’s choosing “gentle” instead of “gently” in crafting one of the two refrains in the poem often receives the attention of those who engage in literary analysis of the work. Since diction is of primary importance in poetry, it must be assumed that Thomas chose one word over the other with careful deliberation and for a specific reason. He would not have employed “gentle” to preserve the line’s iambic pentameter; both words are compatible with the meter of the refrain. Instead, the reason he selected “gentle” rather than “gently” is found in the meanings of the words and how they function within a sentence. In the difference between “gentle” and “gently” lies the heart of the poem—its major theme and the poet’s anguish as he witnesses his father’s dying.
The obvious difference between the words, as often noted, is grammatical, the adjective vs. the adverb. In choosing “gentle,” the adjective, Thomas addresses his father’s emotional state as he dies. “Gently,” the adverb, would have related to how his father experiences the process of dying. In imploring his father not to go “gentle” into death, Thomas is urging him not to be gentle in spirit as he dies. To be made “gentle” is to be tamed by overwhelming force, broken in spirit and powerless to resist. Thus Thomas implores his father to resist being “gentled” by impending death.
As the poem progresses, Thomas’s plea becomes a desperate prayer. He begs his dying father to remain the father he has known and loved. The poem’s first refrain, “Do not go gentle into that good night” expresses Thomas’s fear of his father’s identity, his emotional vitality and will to live, being crushed by the prospect of imminent death; the more powerful second refrain, “Rage, rage against the dying of the light,” reflects Thomas’s anguish as he considers losing the precious essence of his father even before his body dies.