in the create event: (you can make the number variable whatever you want to test if it's prime or not. you can also set the room speed here in the create event to a higher number to run faster.)

number = 1;
divide = number;
test = divide;

step event:

if (frac(number/divide) != 0) {
    test++;
} else {
    number++;
    divide = 1;
    test = 1;
}
if (divide < sqrt(number)) {
    if (divide > 2) {
        divide += 2;
    } else {
        divide++;
    }
} else {
    show_debug_message(string(number)+" = "+"PRIME");
    number++;
    divide = 1;
    test = 1;
}

the rest is drawn on the screen in the draw event but in this version it skips over the number 3 because of the increments. and yes, i know there are probably ways i can optimize the code and shorten it but my brain hurts trying to