The code I used for my "Hacking Pokemon Red to say Hello! (Nintendo Game Boy)" YouTube video - https://youtu.be/Vm4UpRkBD5g

sum.c

#include <stdio.h>
#include <stdint.h>

int main(int argc, char** argv) {
        FILE* fp = fopen(argv[1], "rb");
        if (fp == NULL) {
                return -1;
        }
        fseek(fp, 0L, SEEK_END);
        size_t len = ftell(fp);
        rewind(fp);
        uint8_t* buff = malloc(len);
        fread(buff, len, 1, fp);
        fclose(fp);
        size_t sum = 0;
        for (size_t i = 0; i < len; ++i) {
                if (i != 0x14E && i != 0x14F) {
                        sum += buff[i];
                } else {
                        printf("Skipping checksum\n");
                }
        }
        printf("Sum: %zx\n", sum);
        return 0;
}

Improve the code if you wish, I just wanted something working asap :P

I updated to print out in hex instead of decimal, one less conversion