What Vernon said. Incidentally, there is a way to type in the coordinates but you'll need to type in full commands.
Assuming you are in one of the layout viewers like DESIGNrev, you can enter these commands in the terminal window from which you invoked the viewer. The command to create the clip based on coordinates is "$L create clip $CELL -clip {X1 Y1 X2 Y2}" where $L and $CELL are previously-defined layout and cell handles, and X1 Y1 are the lower-left coordinates of the region, and X2 Y2 are the upper right. Once you have a clip, create a GDS from it with "$L gdsout filename -clipsAll", where $L is the same as for create clip and filename is the name for the new GDS. The commands are documented in the calbr_drv_ref.pdf in the documentation directory. The $L create clip page has an example that includes setting up $L and $CELL. Example 2 in the page on $L gdsout may also be helpful.
There are also ways to save regions of the layout from a batch run (add LAYOUT WINDOW and a Results Database for just that region, for instance) but there is far more overhead with copying all the layers, setting maximum results, and so on. Doable, but not really recommended unless this is something you'll want to re-use on an automated basis.