UPC and GTIN; As general acceptance, they are the same because GTIN (General Trade Item Number) is digit numbers that are the identifier of the product and UPC is the shorten version of Universal Product Code which is used in United States of America or in Canada. So, we can say that they are the same digit numbers.
Why do we have UPC and GTIN then?
We can shortly say that your products UPC code is for these areas only but your GTIN is for global sale. You may also think that UPC is a GTIN embedded on the barcode.