Moving states with a medigap policy

We are about to sign my father up for a medigap G-HD from Mutual of Omaha. He resides in NJ and is within the 6 month enrollment period post 65th birthday. However we are considering him relocating to be closer t us in California.

I have 2 questions for this group in case anyone knows:

1. Would he need underwriting with a relocation?
2. Are medigap policies nationwide, meaning he can use it in any state for any covered treatment?

Thank you in advance -