From what I have read the Dell PowerConnect 6224 switches do not support GRE and therefore you will not be able to do what you are suggesting. Please, correct me if I am wrong?!
You could look to enable, if it isn’t already, IP routing on the switches as they can perform layer 3 functions. With this enabled you could have equally weighted routes to each of your provider routers LAN IP addresses which would resolve your load balancing issues but it certainly wouldn’t be resilient.
For resilience you would need to look at altering the configuration of the routers as well and use another protocol like GLBP between them and from your switch perspective route to the virtual IP configured between the 2 routers. The router themselves could be configured to track external routes or line protocol in order to stop forwarding in the event of a failure.
It can be a challenge to set up if you don’t have access to the routers though 🙁