You can uninstall or removes an installed apng2gif package itself from Ubuntu 16.04 LTS (Xenial Xerus) through the terminal,
$ sudo apt-get remove apng2gif
If you would like to remove apng2gif and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove apng2gif
If you use with purge options to apng2gif package all the configuration and dependent packages will be removed.
$ sudo apt-get purge apng2gif
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove apng2gif