Make a script for Google Sheets or standalone app for CSV merging

Hi!
I need to make a script, standalone app or anything that will work for my case. I need to merge two CSV where first file have data structure like:
GTIN CODE, QUANTITY,SKU
5061122031633,1,DZ 1327
5061122409799,1,AB 3003
5121145047774,1,AB 3318
5061145510168,1,AB 4729

and second CSV file:
GTIN CODE, QUANTITY
5121145047774,6,
5061145510168,4

First one is a backup from a warehouse list, and second one is a file that is made by barcode reader. I need a script or anything like that, to merge these two files. It should work like that:

- it should find the same GTIN numbers in file 1 and file 2, and then ADD it's quantity from both files to one. GTIN numbers will be always in a random position, and file 2 will always have less positions than file 1, because we are not going to scan whole warehouse for once.

The end file should look like:
GTIN(EAN), QUANTITY(updated), SKU
5061122031633,1,DZ 1327
5061122409799,1,AB 3003
5121145047774,7,AB 3318
5061145510168,5,AB 4729

If this will help, I can just delete the SKU from 1st file( thats no problem), and in end file it's not mandatory too.