How to add Gallary in wordpress:
1) Open your posts and Upload all the images in a bunch.
2) Click on save all
3) The window will be closed automatically. Open the windows again and you will find the gallery option there.
If this does not work, deactivate all your plugins and also check the permissions of your wp-contents/uploads folder
Also, make sure you are not using any other gallery plugin, especially NextGen because it may conflict with the wordpress’s default gallery.