By default, documents are opened in new tabs inside the same gedit window.
Move a tab
There are several means to move a tab to another location, to organize the files you edit in the way you like.
You can right-click on a tab title and choose the desired option.
Keyboard shortcuts are available too.
Drag and drop tabs
Another means is to drag and drop a tab:
Click and hold the mouse button on the tab title.
Move the tab to the desired location.
Release the mouse button.
Drag and drop locations
To re-order tabs within the same window
Drag and drop the tab along the tabs titles.
To create a new window from an existing tab
Drag and drop the tab out of the gedit window.
When gedit is maximized, drag the tab onto the top bar of GNOME Shell at the top of the screen.
To move a tab from one gedit window to another
Drag and drop the tab to the other window, either within the text area, or within the tabs titles to choose the exact location.