If the generator is not an SDS, the system grounding remains the same, using the service GES. The only way you would need a ground rod was if you called it a "structure" IMHO.
250.35 still requires an equipment bonding conductor sized to 250.102(C), which tosses you back to 250.66 based on the size of the ungrounded conductors to the generator.
This does bring up an interesting conundrum. The smallest EGC in 250.66 is 8ga (but no larger than the size of the ungrounded conductors in associated rules).
#10 cable, no problem, the EGC is #10. #8 cable is a problem tho since the EGC will usually be #10.
