I think the GP reset is positive, but instead of making it a recurring reset, I suggest that you consider to change how the decay system works for example a 4 star currently loses 98 GP no matter how much GP he/she has, if we take the base GP requirement 3,064 and then add an additional decay of 1% of the excess GP, in my case with about 43k GP that would mean that the daily decay would be an additional 400 GP, this means that if would be very hard to "park" a character with 40k or 100k or more, as the decay would be checked through this mechanism and it would be practically impossible to reach more than about 100k even if you take r1 reward at every siege and capture/defend every fort.
As you have the data on how many GP everyone has (and their ranks) such a proportional GP decay would be quite easy to implement.
On a second notion this talk about that you can get 900 GP at siege is dishonest at best, first only 4/7 sieges allows for this even theoretically and then 900 GP implies that you have top tier contrib at all forts AND, and this is important, your faction defends/wins all contested forts. When did an ely get 900 GP at siege the last time? In practice if you have some gear and dont have an x-form on ely side and is active you typically can get tier 2-3 contrib, tier-1 is very rare i would say at least at more than 1 fort. This means rather maybe 200-300 GP than 900.