How do you view all the GCs in the forest?

C:\>repadmin /showreps
domain_controller 
OR 
You can use Replmon.exe for the same purpose. 
OR 
AD Sites and Services and nslookup gc._msdcs. 
To find the GC from the command line you can try using DSQUERY command. 
dsquery server -isgc to find all the gc's in the forest 
you can try dsquery server -forest -isgc.