What makes me confused about naming scheme in AMD is there are Generation, Architecture, and Graphical Processor.

no. GCN is architecture, ala Zen. Polaris is generational marketing name ala Summit Ridge. That is all. GCN covers a lot of marketing names from HD 7000 to Vega.

Meanwhile Nvidia only has straightforward naming scheme like 30 series is Ampere, 20 and 16 series is Turing, 10 series is Pascal, 900 series is Maxwell.

it's not that different. Nvidia has a core architecture like GCN but don't use it in marketing slides, you just see the revision's marketing name ala Ampere. they don't develop a whole new arch each generation.