Hello
For what im looking you got 2 internets(the one for guest that you got attached directly to the Controller, and the other port that is attached to the internal network... and well the internet of the internal users is though that port...
And you want to send vlan 500 which might be the guest users lkike i said....
If that your scenario i dont think what you want to do willl work but you got other options which are
1-Put a default gateway pointing to internet, and natting the guest users with no issue. For the internal users you dont need to do anything as their default gateway is the core switch and not the wireless controller... if they are going to internet... they willl go to their gateway first which is the switch core and then the switch core willl send them to internet.
Now you will need to add some routes so you can manage the Wireless controller from the internal network(will explain that if you dont understand if this is the case)
2-Best option and most secure one. Put the default gateway like you got it right now. For the second internte which is the one for the Guest users connect it to a small firewall, which will be the default gateway of the Guest users.
In this case their default gateway will be the Small firewall... AS that is what you will distribute in the DHCP you will tell them well your default gateway is the small firewall... and the small firewall will nat them out. For the internal users the switch core will be their default gateway.... And you will have not any problem managing it from your internal network...
I personally prefer option 2
I dont like connecting my wireless controller directly to internet...
If you select option 1 you will have issue with RAPS.... and VIAs if you point them to the corporate public network(not the guest network public ip address)
The packet in this case will go in through the corporate public ip address and will try to go out through the guest public ip addresss and it wont work.
IF this is not the case please explain your case with more detail to see if we can help you.
Cheers
Carlos