How can I change the network distance to the specific gateway?
In the routing > ospf > interfaces tab:
add a new Interface, choose the interface you want to use as the backup, and set the cost to something higher, such as 100 or 1000. (in general, 1 = the fastest link possible such as 10Gbps, 10 = gigabit link, 100 = fast-ether, 1000 = 10Mbps)
Be sure to do this on both ends of your backup link
If the destination route you're interested in is an external route (redistributed into OSPF) then you may not actually see the distance change. Type 1 external routes will increase the distance metric in each router along the path, but type 2 external routes will not.
Type 1 means you should consider the cost of reaching the gateway AND the cost beyond the gateway. (thus your internal path cost is added to the original metric set by the ASBR)
Type 2 means you should only consider the cost beyond the gateway - so the path costs don't get added along the way. However, if there are multiple paths to reach the best type2 gateway, the path cost will be used to determine the best next hop to reach the best gateway.