An important advantage of using GUI data-entry objects is that you often can control what users enter by limiting their options?

Yes, usen check boxes, text boxes, or other can control the type of entry

Explanation:

Using a check box, you can force the user to select just options presented in the list.

If the data to insert is a data, its usefull to use a calendar widget to avoid bad data input a to make sure the format is correct.

In a text box, yo can you if the typing text is numeric or alphanumeric.

Basically, the GUI is simple to use and can restric the data entry only to the preassigned options.