This is to reduce maintenance burden. A GIF representation of a web
page can be produced by converting its PNG capture. There are numerous
third-party tools which can do the job better (optimized palette,
minimized loss, etc).
https://github.com/ariya/phantomjs/issues/12480
As it seems that project Mongoose has been relicensed from MIT to GPL2. The link provided is leading to the new licensing agreement but as today version 3.1 of Mongoose is used which is licensed under MIT therefore correct reference to a license should be provided.
https://github.com/ariya/phantomjs/issues/10718