leaky arr

Your code has a bug.
Let say g=3, m=3, n=4
Result: [0, 3] // Should be[0,0]

Let say g=5, m=5, n=6
Resutl: [0,6] // Should be [0,0]

Your function will getting problem when g=m and n > m + 1 (or more)
Your problem code is here:

if (candidates[1] - candidates[0] == (long long)g) {
return candidates;

For the first iteration
candidate[1] will fill with first prime number
but candidate[0] is 0.
So, when first prime - 0 == g (your code wrong here)
// you should start compare candidate[0] and candidate[1] after these two values are filled

The description it too simple until made me confuse. Is the function requires remove all the duplicate values and return that array/list or need to count how many values are distinct and return that number. If just need to tell the number of distict values why made this function to return a pointer.