Walt Crawford has published "The Gold OA Landscape 2011-2014" in Cites & Insights: Crawford at Large.
Here's an excerpt:
This issue consists of an excerpted version of The Gold OA Landscape 2011- 2014, published September 10, 2015 as a PDF ebook for $55.00 and on September 11, 2015 as a paperback book for $60.00. . . .
This book represents the first overview of essentially all of serious gold OA—that is, what's published by the journals listed in the Directory of Open Access Journals. I believe it's important for all OA publishers and for many libraries and OA advocates. If it does well, or if there's some form of alternative funding, I'll continue tracking the field in the future.
The issue—starting with the first numbered section below—includes a little more than one-third of what's in the book (a little more than half the text, but none of the 69 graphs, and probably less than half of the many, many tables), I believe these excerpts are useful on their own, and enough to provide a reasonably good picture of gold OA in 2011-2014- but they're not the whole story. For that, you'll have to buy the book.