Where can I find all the GuiXT "enter" posibilities?  e.g.: Enter, Enter "/n", Enter "/nF12" , etc, etc.

With "Enter" you can enter all values that a user could enter directly in the OK-code field.
 /Nxxxx  where xxxx is a new transaction code;
 /x  where x is the number of a function key (e.g. /11 for a "Save");  and
=ffff  where ffff is a program defined function.

General documentation comes up when you press F1 on the OK-code field.
For other functions, I would use the recording option in GuiXT.
You can also use the F1 key on menu items and pushbuttons in order to get the internal function code.