If there is mix between number & string you will likely end up convert them into string. Do you store string or number on instance variable? Assume that instance variables numbering is what you are after;
-straight expression would be: gAMEoBJECT.IVnumber_1 for example
-You probably expect gAMEoBJECT."IVnumber_"&GlobalVariable
to work but no it's not work that way.
But if it just a string you could retrieve it's ObjecTypeName and put global variable value:
+ System: On start of layout
-> System: Set GlobalVariable to choose(1,2,3)
-> Text: Set text to gAMEoBJECT.ObjectTypeName&"."&"IVnumber_"&GlobalVariable
That will display "gAMEoBJECT.IVnumber_1" or "gAMEoBJECT.IVnumber_2" or "gAMEoBJECT.IVnumber_3"