Most Gridcoiners run only PPS Sieve subproject on PrimeGrid because it is very well suited for new Nvidia GPUs and it brings far more credits than any other subproject (which in turn brings more GRC). You can't discover new prime-numbers by running PPS Sieve, you are just doing preparation work, searching and discarding lots of non-prime numbers, so that other subprojects have less to do, focusing on numbers which look like promising candidates for primes.

More about sieving here:
https://steemit.com/mathematics/@cryptohazard/the-prime-numbers-from-mystery-to-reality