Like most rumors related to live-action adaptations in general and Gintama in particular, I’d take this one with a boulder-sized grain of salt until more concrete information can be obtained.
Japanese fan site Otapol is reporting that (according to unnamed official sources close to the production), a live-action Gintama film starring actor Shun Oguri (who starred in the 2016 live-action TerraFormars film and the 2014 live-action Lupin III film) as Gintoki Sakata is currently in the works.
The project is allegedly shooting for a release date in 2018.