One of the handiest tools is the Clipboard. 
in the toolbar.
You can place a citation (or a source) here so it will be available for use during your Gramps session. You can then drag-n-drop from the clipboard to wherever needed. You can drag a place record to the clipboard ready to be dragged into an event record. Or drag an event record to be shared with other people.
The clipboard floats and available for all views.
Not to be confused with the Clipboard Collection that can be added to the side-and bottombar of any view. These clipboards can hold items between Gramps sessions.
Here is a recent discussion about clipboards.