Hello Harshit
Fundamentally, reference bandwidth should always be the same on all routers taking part in OSPF. Now if you choose to use 1000 Mbps as the reference bandwidth or 10000Mbps, or even 100 Mbps, it doesn’t make a difference what the speeds on your interfaces are, as long as all routers are choosing costs based on the same reference bandwidth.
Now in the case where you have a GigabitEthernet interface connected to a FastEthernet interface, it shouldn’t make a difference. This is because if these two interfaces were connected, the GigabitEthernet link would revert to to 100Mbps, and the reference bandwidth used would refer to that speed rather than the GigabitEthernet speed. So even if you use 100Mbps or 1000Mbps for the reference bandwidth, the result would still be good.
Rule of thumb: always have the same reference bandwidth on all OSPF routers.
I hope this has been helpful!
Laz