One Piece: Devil Fruits
You will die regardless of having eaten a fruit or not, except when in Buffalo Lake.
If you eat a Devil Fruit whilst having consumed one, it will remove your power and rid the fruit. To prevent this, eat a black fruit to remove your current Devil Fruit powers and then consume the new one. Remember that the black fruit will kill you, so don't have the desired fruit in hand.
Gum Gum Gatling does break after you die, yes.
Again, I will try to fix this but it may take some time.
Project put on the backburner, won't continue development for a while.