I can confirm the Gigabyte GeForce GTX 750 Ti (GV-N75TOC-2GI) requires a 6-pin connector, without that it will refuse to boot
http://imgur.com/a/tAgXd
It's strange that Gigabyte doesn't mention this on the specification page
http://www.gigabyte.com/products/product-page.aspx?pid=4948