As you all know each area in an OSPF AS should be connected to the backbone area - area 0.
Basically in order to do that we are using virtual-link to interconnect between areas but there is another way to do that using GRE.
Following Narbik Advanced CCIE R&S Workbook 2.0 here is OSPF LAB 6:
In order to connect area 3 (loopback interface on R1) to area 0, we need to create a link between R2 and R1.
We can use virtual link and configure the following on the routers:
R1(config-router)#area 1 virtual-link 184.108.40.206
R2(config-router)#area 1 virtual-link 220.127.116.11
Where 18.104.22.168 and 22.214.171.124 represent Router-ID for each router.
or we can configure GRE tunnel between both routers, take note that GRE does pass broadcast and multicast and often use with IPsec in order to provide this abilities which IPsec doesn't support.
So configuration goes as follows:
R1(config-router)#network 126.96.36.199 0.0.0.0 area 0
R1(config)#interface tunnel 1
R1(config-if)#ip address 188.8.131.52 255.255.255.0
R1(config-if)#tunnel source 184.108.40.206
R1(config-if)#tunnel destination 220.127.116.11
R2(config-router)#network 18.104.22.168 0.0.0.0 area 0
R2(config)#interface tunnel 1
R2(config-if)#ip address 22.214.171.124 255.255.255.0
R2(config-if)#tunnel source 126.96.36.199
R2(config-if)#tunnel destination 188.8.131.52
pay attention that we should advertise the tunnel ip in area 0 else OSPF won't advertise this link and wouldn't be able to exchange routing information.