> If my SEPM is in latest version 12.1 RU3 and my clients are still running SEP11. Can my SEP 11 client still get definitions from my SEPM?
yes
> If my SEPM is 12.1 RU3 and I promote one GUP (with SEP 11 installed). Will my GUP able to update clients with SEP 11 version?
Yes, SEP 11 GUP can provide updates to SEP 12 and SEP 11 clients, however you need to work on the upgrade plan for the GUP as well.
> If my GUP is running SEP 11 client. Will it be able to update both SEP 11 and SEP 12 clients?
Yes, SEP 11 GUP can provide updates to SEP 12 and SEP 11 clients, however you need to work on the upgrade plan for the GUP as well.
> Does GUP update all types of definitions such as NTP, PTP or only AV/AS?
Yes