How can I make that when the user enlarges one row all rows will be resized to the same height?

You should call

   GetParam( )->EnableTrackRowHeight(GX_TRACK_ALL); 

See the class reference for CGXGridParam::EnableTrackRowHeight for further modes.

Take also a look at the FAQ " I have set EnableTrackRowHeight(GX_TRACK_ALL), but some rows are not getting resized at all. What is wrong?"