Hello Edson,
happy chrimas to you too
GLBP is a good solution if you have many clients on the internal LAN because it works based on ARP:
when a client makes an ARP request to resolve the VIP address of the gateway the answer given by the AVG (Active Virtual Gateway) varies over time providing the MAC addresses of the current AVF (Active Virtual Forwarder)
more info here
http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_glbp_ps6441_TSD_Products_Configuration_Guide_Chapter.html
if you have only a firewall on the internal LAN GLBP is not enough to provide outbound load balancing and you need to use two default routes using two HSRP groups (where the active is R1 on group1 and R2 on group2) as next hops for the two default routes
you need also to advertise your subnet(s) on both ISP links
Hope to help
Giuseppe