How do I stage all files at once in Git Gui?

Yes,

Select the items (select top one hit shift, select bottom one) and hit CTRL T

Or go to commit -> stage to commit

Quite late, but one method is to add an option in the 'Tools' menu.

1. Click 'Tools'>Add...
2. Name it whatever you want (I chose 'add all')
3. In the 'Command' field, type git add *
4. Optionally check the boxes to remove a dialog window, output window, and global configuration.
5. Click the Add button in the bottom right.

Now, click Tools>Add all and all changes will be staged. Hope this helps!