Re: Can I select a Parent within the same list and also another list?
What you are describing looks like what we call a "Composite" or "Balanced" hierarchy
So you could definitely set up the list that way.
However, you mention the second list which would be made up of G2 and G3
You can maintain that list as a flat list and associate each member with the relevant G2 or G3 member by having a line item formatted as G2 and G3 respectively. If you use the same code for the member of G2, G3 and the flat L2 list the association is easy and can be managed with a FINDITEM formula
Alternatively, you can create a module dimensioned by G2 and have a line item formatted as the flat L2 list. And do the same for G3, again using FINDITEM to map the list members together
That way you can then use SUM or LOOKUP to transform and /or aggregate data between the structures
How you structure the lists will depend on the logic and calculations you need, but the thing to bear in mind, is that Anaplan does not need to have hierarchies for everything and doesn't need hierarchies to aggregate data - we can use SUM on formatted line items to achieve the same result