As far as I know, Pixel Gun World has been officially discontinued which means that you will no longer be able it download to from official sources. Furthermore, its servers have been shut down, so even if you find a source to download the game from, the only way to play it would be to gain access to a private server or create one for yourself.
So, if you found a website that allows you to download the game, make sure that you're not being led to some kind of weird page instead of a legitimate download and use the VirusTotal online service to scan the installer before running it.
