Tesla designs and manufactures battery packs. GM is on record saying that it took them a long time and a lot of money to develop the battery pack for the Bolt.
GM didn't develop the battery pack for the Bolt. The battery cells, battery pack, charging systems, motors, computers and infotainment systems in the Bolt are all designed and manufactured by LG Chem.
Tesla uses Panasonic cells to make their batteries[1]. I'm sure Tesla has some circuitry/electronics to control the cells, but it's not like the key battery technology is all Tesla's.
The Gig Factory allows Tesla to ensure they have enough batteries for their car fleet so that they don't suffer from shortages as other MFGs put a demand on suppliers.