These sites mostly aggregate others, providing you a browsing-friendly entry point. There aren't many left ...

• KindlyRat's GWBASIC Page: Originally on Geocities until it was shut-down, KindlyRat was the place to go. In fact, the references to the old URL (http://www.geocities.com/KindlyRat/GWBASIC.html) are scattered far and wide (as can be for an obsolete technology).

Learn where GW-BASIC fits in the annals of history as well as alongside other BASICs.

• Microsoft BASIC Version Information.

Guides to learning GW-BASIC or other forms of BASIC.

• The Secret Guide to Computers - Other Basics:  Long-scrolling page quickly describing different versions of BASIC, including GW-BASIC and Visual Basic for DOS. As educational information it is most useful for those moving between various versions of BASIC, but for new programmers it may be of limited value.
• Last Book of GW-Basic by the crusty and elusive Joseph Sixpack. Just do a search for the title to find an archived copy since it was on Geocities and Yahoo! destroyed it. I'm currently being entertained by it and am considering making my own archive copy (e.g. ad free!).
  
• BASIC Training: A GWBASIC tutorial by Steve Estvanik and converted to HTML by Thomas Antoni. This is thorough with the fundamentals of the language but only briefly touches graphics and sound.