Is the key safe if attacker know the encrypted and decrypted message? Given an attacker know both the encrypted message (+ an iv) and the equivalent decrypted message, is it possible for him to craft a new encrypted message that could result in a desired decrypted message? I'm using aes-256-gcm algorithm specifically.
For any proper encryption algorithm, it is safe. The type of vulnerability you are describing is a key recovery attack or a forgery attack involving known plaintext-ciphertext pairs. Only very insecure ciphers are vulnerable to that attack, and strong, modern ones like AES are not.