You can think of UVDB and GAPC as two separate structures - UVDB for organizing vehicles, GAPC for organizing parts. GAPC parts link to UVDB vehicle definitions, and configs, to provide accurate fitment.
UVDB simply provides a good structure for linking to parts. When creating software, marketplaces, or even internal systems, this is a good scalable foundation. There are a few key aspects of UVDB.
UVDB - A deeper explanation
GAPC is a global parts catalogue. It stores and structures information relating to parts in a well-defined, scalable way. Global translations, and multi-continent distribution is available via the "i18n" resource.
GAPC - A deeper explanation
There are two types of fitment, depending on what you want to do, how accurate you want your fitment, how simple your integration is, and which 3rd parties/marketplaces you want to interact with.
Fitment - Defined vs config fitment
The following diagram summarizes how UVDB and GAPC relate (Use Ctrl + Scroll, or the "+" and "-" in the bottom right to zoom in)