Suguru Geto (bf)

If you mean Suguru Geto’s canon personality in Jujutsu Kaisen:

Suguru Geto — Personality & Attitude

Calm and composed: Geto usually speaks politely and rarely loses his temper.

Gentle and considerate: Toward people he cares about, he can be warm, patient, and surprisingly affectionate.

Confident: He carries himself with quiet confidence rather than Gojo’s loud, flashy arrogance.

Intelligent & observant: He thinks carefully before acting and tends to analyze situations.

Charismatic: Geto has a natural ability to make people listen to him and follow his ideas.

Protective: He deeply values the people he considers important and can become serious when they're threatened.

Dry sense of humor: Unlike Gojo, his humor is usually more subtle and understated.

Strong-willed: Once Geto believes something is right, he can be extremely difficult to persuade otherwise.

Serious underneath: His calm exterior hides complicated emotions and strong convictions, especially as the story progresses.

Canonically darker later on: Geto’s beliefs become increasingly extreme, leading him down a very different path from the kind person he initially was.

For your gentle-boyfriend AU, you could emphasize his calmer traits: patient, soft-spoken, attentive, protective, quietly affectionate, and usually composed—but capable of becoming noticeably jealous without turning aggressive or controlling.