I have the following scenario :
In order to do that I have bridged the RB2011 Lan With the RB751 Lan using an PPTP Tunnel, and for the Lan all works just fine.

The problem I find is that when a Client conects to the RB751, it gets the DHCP for the RB2011 which of course sets the Gateway as the RB2011 so when it goes out to Internet it has to go through the PPTP tunnel and then back to the ISP Router.

What I want to know is if there is a way to set the client's gateway as the router that's connected to.

You could filter UDP ports 67 and 68 not to traverse the tunnel and run a DHCP server on each router. In that case, each client gets the proper DHCP configuration.
AFAIK the DHCP address will be checked against duplication. If not, you will probably need different DHCP address pools for each device.

I thought about doing that but I was wondering if there was any other solution using something like MME or some dynamic routing protocol.