sql in and out query Writing way ( mySql )

sql in  query Writing way :

 SELECT *FROM rbs_user

  SELECT e_mail FROM rbs_user WHERE user_id IN(1,2)//it is correct,it is auto increment id
  SELECT e_mail FROM rbs_user WHERE ud_user_id IN('gp101','gp102')


  SELECT e_mail FROM rbs_user WHERE ud_user_id IN(gp101,gp102) //Wrong way


 SELECT *FROM rbs_user