Keep in mind your company may have created a few very nice GUI's to be used in your clients systems. The programming to make the GUI work with their equipment is what the client might be paying for, thus they do not own the rights to your GUI, just what is underneath the buttons.
Of course if you are being paid to create something completly unique for a client, that may have family pictures or other personal information worked into the design, then sure, that should belong to the client.
Based on what some are saying here, if the client owns the GUI, then you have no right to use it on your next project either...