Before creating your first gallery it is advisable to check the Flexi settings that whether auto configured settings are correct or not.
Flexi is different from other plugins, here each post acts as individual post or image. It will not create a set of galleries. Later with the help of shortcode we group the required post/image to create galleries based on album, tags, users etc.
You can create your gallery from back-end and also from front-end.
This approach is used when administrative don't want to give access visitors to submit post from front-end. To post it is as same as creating a wordpress posts. Just give necessary title, description , image and choose appropriate category as it has been asked.
This page can be generated in 2 ways. 1st with the help of Guten Block which is very easy to use and 2nd with the help of form shortcode which is already generated as soon as plugin activated and can be found at pages.
This also can be generated with Guten Block & Gallery shortcode which is ready to use and already generated. Check in list of pages.
Use above form to submit images/video which will be displayed at gallery.