A lot of the research papers and/or presentations are also available online if you search google using the terms I noted before. Best thing to do is if you know what topic you want to learn such as “Microfacet Models for Refraction through Rough Surfaces” is to search for that term along with the word “research” or “presentation” or “paper”.
You don’t need to subscribe to GDC, just search for what you need online and look for PDF files, it will all be there!