Building a SaaS for product data using GTIN and GDSN

I wanna build a SaaS solution to connect invoice data to the GTIN to track the articles a company buys and sells.
My problem is that I am a newbie, when it comes to GTIN. Does anybody here has experience with GTIN and GDSN and could explain to me what kind of data I can get to every GTIN and if there are paid services that provides this Data.
I of course googled and chatGPTed, so I have a bit of an idea, but would love to hear from an expierenced human.
Thanks