If you complete IGETC or CSU-GE before transfer, then you will not need any more GE courses after transfer.
If not, then any courses you complete before transfer will be applied to GE categories at the CSU that they match up to, but then you will have to take additional GE courses in the categories that you have not fulfilled.
http://www.assist.org/web-assist/report.do?agreement=aa&reportPath=REPORT_2&reportScript=Rep2.pl&event=19&dir=1&sia=LACC&ria=CPP&ia=LACC&oia=CPP&aay=15-16&ay=15-16&dora=MECH+ENGR
http://www.assist.org/web-assist/report.do?agreement=aa&reportPath=REPORT_2&reportScript=Rep2.pl&event=19&dir=1&sia=LACC&ria=CSUN&ia=LACC&oia=CSUN&aay=15-16&ay=15-16&dora=ME
Looks like you would be missing 25 quarter units (= 16+2/3 semester units) of major courses at CPP, and 8 semester units of major courses at CSUN. You would have to take them as “catch up” courses after transfer. It may be best to have IGETC or CSU-GE done before transfer, so that you can be ahead of schedule on the GE courses, leaving more room in your schedule to take the “catch up” courses while minimizing the number of extra quarters or semesters needed at the more expensive (than CC) CSU.