Code 128 is a very versatile type of bar code (symbology) that can encode any of the symbols on a keyboard. There are 128 in all.
GS1 adopted a variant of Code 128 for use when there was a need to put into a barcode a complex string of numbers that included the GTIN of a product and additional data such as batch number, use-by date or such like.
The variant is now known as GS1-128 but because GS1 was known by different names throughout its history the previous names of the variant will still be found in software and literature. It may be called;
All of these names effectively mean the same, GS1-128
Do not use ordinary Code 128 to encode GS1 numbers. The result will be a barcode that will not work.