Once you reach Level 80, you'll be around 3k GS. You can buy the Level 80 stuff from BS/LW/Tailoring to get better gear. To unlock heroic RDF, you'll have to reach ~3.4k GS. You'll be able to join Naxx at this point. After that, you can start farming EoTs to buy the set gear.
At around 3.7k GS, you'll unlock PoS, FoS which drop ilvl 232 gear, the same level gear that you can buy with EoTs/that drops from ToC10 nm.
Once you've obtained most of EoT gear, you'll be past 4.5k GS. At this point, you'll be able to join ToC10 nm to get the remaining gear. This will push you to 5k GS and allow you to do most of the raids with the exception of ICC.