As comic book movies get adapted for the screen, there are always going to be decisions to be made regarding what elements from the source material to use, and what elements to change. In the case of the Guardians of the Galaxy, James Gunn wasn't only deciding on the small details but also what members of the team itself to include. One element that even James Gunn admits he liked, Peter Quill's element gun, was something that didn't make the cut, though the director has a solid explanation for why.
In the comics, as well as the Disney Guardians of the Galaxy animated series, Peter Quill has something called an element gun, a blaster that can reproduce the four elements. Recently a fan asked why the element gun does not appear in the films, and director and writer James Gunn explained that, while he thought the weapon was cool, it was just one more thing that the movie would need to explain, and he ultimately felt it was one thing too many. According to Gunn...
Peter Quill's standard guns (they're his thing) in the Guardians of the Galaxy films look like the element gun from the comics, but they are fairly standard laser guns. They shoot some sort of energy blast, the sort that anybody who has ever watched a science fiction action movie has seen and thus doesn't need to have them explained.
The element gun, as it shoots things that cover the four elements, would be somewhat unusual and, as James Gunn points out, that would need to be dealt with. Nobody else has a gun like this, we haven't seen it anywhere in the Marvel Cinematic Universe before, and that would need to be explained. Since every movie has to do its share of explanation for its characters and universe, the gun just adds more that would need to be explained, and the director wanted to make his story as streamlined as possible, so things that didn't need to be there, like the element gun, were dropped.
As the initial tweet points out, the element gun does appear in the animated Guardians of the Galaxy series. While the animated series is, in large part, in existence because of the film's success, it tells a separate story and isn't an adaptation of the MCU movies, so it does a lot differently.
But who knows? Now that the rest of the Guardians universe has been explained, maybe the element gun will appear in Guardians of the Galaxy Vol. 3. Assuming of course that movie won't have lots of new characters to introduce or a complex plot that will need time of its own. Yeah, probably not going to happen with the gun.