Don't know will GNIIT help in ur MCA entrance exam but I'm sure that it'll help a lot in ur whole MCA course.
I'm doing MCA from IGNOU and there is no entrance Exam, thats why I dont know about entrance exam! But doing some computer course like GNIIT helps a lot in MCA coz I hv seen many students who didnt know nething (or knew a little) about computers having troubles...
I too did DNIIT course from IGNOU and it was good...