Introducing the Gallery widget

In the last major release we introduced Media Widgets for Images, Video, and Audio. Per that dev note: WordPress 4.8 includes media widgets (#32417) for not only images (#39993) but also video (#39994) and audio (#39995), on top of an extensible base for introducing additional media widgets in the future, such as for galleries and playlists.
Now in the upcoming 4.9 release this Gallery widget (#41914) has just landed in trunk in [41590]. Just as users can add galleries to their post content they too can add galleries to their sidebars. The media widgets are being developed with Gutenberg in mind, as widgets are essentially proto-blocks. Gutenberg has ported the Categories and Recent Posts widgets as dynamic blocks so that users can add to their posts what was formerly restricted to sidebars. In the same way, the media widgets are allowing for content that was formerly restricted to post content to also be available for addition to widget areas. As Gutenberg matures, widgets are planned to eventually transition over to use blocks, and the widgets for images, video, audio, and galleries will be able to be migrated over at that time. In the mean time, the user should not have to know there
Source: https://managewp.org/articles/16191/introducing-the-gallery-widget

source https://williechiu40.wordpress.com/2017/09/25/introducing-the-gallery-widget/