Hello aspirant!  Let me to inform you that according to the previous years cut off rank for isro and drdo, you must have a gate rank below 150 to get a interview call from these esteemed psus. For both of the psus you must be among one of the top ranked students.

For joining ISRO after BE you should have 65% average of all semesters.

If you fulfill the criteria mentioned above, then you can apply through ICRB for a scientist/engineer post. The selection procedure includes the MCQ test, interview followed by skill test. If you clear all you'll get into ISRO.