Gundlach is being vocal about how the 3% yield level for the 10y is a key level that could send stocks down. The issue is that he is using a variation of the 'Fed model' to predict stocks, which historically had serious deficiencies compared to other valuation tools like simple PE ratios
Cliff Asness does a great job of explaining why one can't just look a nominal bond yields when assesing where stocks are headed in this paper
http://faculty.som.yale.edu/jakethomas/otherpapers/Asness.pdf
That paper should be required reading to anyone investing these days. There is so much nonsense floating around in terms of comparing stocks to bonds or talking about discounted rates of stock cash flows and the paper just debunks it all. Here are some selected quotes
Cliff Asness does a great job of explaining why one can't just look a nominal bond yields when assesing where stocks are headed in this paper
http://faculty.som.yale.edu/jakethomas/otherpapers/Asness.pdf
That paper should be required reading to anyone investing these days. There is so much nonsense floating around in terms of comparing stocks to bonds or talking about discounted rates of stock cash flows and the paper just debunks it all. Here are some selected quotes