From the first episode of One Piece Luffy chants "Gomu Gomu No" to start his attack, it is also seen in other devils fruit users too. Is there any particular reason for it?
All fruit users in one piece can use his fruits powers freely, the reason for Luffy and every other to put a name to all actions and shout it in battle is only a trope of the shonen style anime.
This fact can be checked by seeing Luffy "loosing" his head when he ate the fruit, or any logia user making his body ethereal without saying a word. Or moments when characters "invoke" the name after doing the action itself.