There has been speculations, rumors and plans to make a Gears of War movie for a very long time, and it wasn't until recently it was confirmed that Netflix will be the ones creating this (and an animated series).
But there were actually other Hollywood movie companies interested way before this and New Line Cinema actually bought the rights back in 2007. In the end it never materialized, and while visiting Xbox Expansion Pass podcast, franchise creator Cliff Bleszinski explains why it ultimately fell through:
"People wanted to see the chainsaw, they wanted to see Locusts getting cut in half, and that would make it rated R. [Executives] wanted to make a PG-13 version of it, and I'm like 'No, screw you guys.'"
Another problem, according to Bleszinski, is there were way to many people involved, something Epic considered to be an issue.
Hopefully Netflix will be able to deliver a great Gears of War movie, and we have to assume it will be very rated R when it arrives, presumably 2025 at the very earliest.