condition :
    #( relation expression expression)
  ;
 
relation:
    GT
  | GTOE
  | EQUALS
  | NE
  | LTOE
  | LT
  ;

antlr sem.g
ANTLR Parser Generator   Version 2.7.4   1989-2004 jGuru.com
sem.g:46:8: rule classDef trapped:
sem.g:46:8: unexpected token: relation
error: aborting grammar 'Sem' due to errors

Instead you have to:

condition :
    #( GT expression expression)
  | #( GTOE expression expression)
  | #( EQUALS expression expression)
  | #( NE expression expression)
  | #( LTOE expression expression)
  | #( LT expression expression)
  ;