I'll say what I think, and you say if you agree, but curses and spells are exactly the same thing, just with a different purpose, everything is magic, just applied in a different way, magic can be used for attack and defense, for protect or destroy, a spell can be used for good or for evil, thanks are blessings because they enormously benefit the bearer, giving him enormous magical powers, and even immortality, as it is said that as long as an archangel has his grace he will live, but a curse is always for evil, it's something you do to harm your opponent, put some curse or restriction on him, doing something against his will, something that isn't just about hurting or killing your target, but it will furthermore, Meliodas and Elizabeth were forced one to reincarnate and the other to be immortal, which should be good, but in this context it is very bad. That's why I think Gowther's spell also fits a curse, as it was meant to curse Mael, a punishment for killing the one Gowther loved.