Each +1/+1 counter put on the target creature is a separate event. Replacement effects may affect each of these counters being put on the creature.

The state-based action that removes matching +1/+1 and -1/-1 counters won't check until after Lifecrafter's Gift finishes resolving. It's possible put a +1/+1 counter on a creature with a -1/-1 counter on it and then give it a second +1/+1 counter.

If there is a +1/+1 counter on a noncreature permanent you control, such as a Vehicle that isn't crewed, it won't get another one from Lifecrafter's Gift.

If the target creature is an illegal target by the time Lifecrafter's Gift tries to resolve, the spell doesn't resolve. No creatures receive +1/+1 counters.