Q.How can we calculate the GM of the vessel if the loadicator fails?

This is the main reason why the Cargo Plan made should be as realistic as possible and more detailed. The duration of each calculation statge should be as short as possible. There are mates who make cargo plans for Commencement- 50% Cargo loaded - Completion stages. During the voyage from load port to disport or vide versa, there is sufficient time availiable for the mate to do the calculations at 2hrs interval (preferably). This will be very helplful for the Duty Officers to follow the maintain the cargo watch as per the Cargo plan. 

Now incase the Loadicator fails, the Cargo plan will be the first point of reference to ensure you are loading within safe parameters. The vessel's drafts and the cargo tank soundings will indicate to you that whether you are following the cargo plan. And since you have calculated parameters like drafts , Stresses & GM, you will know that you are within the safe parameters zone. 

Also it is very important to install the Lodicator software on another PC (preferably a Laptop) as a back up option incase the Main lodicator fails. 

Lastly, the Stability booklet has  form for quick calculation of the basic parameters. Locate this form and make a photocopy of it and display it in the CCR with the procedure. Main priority for mate is accurate loading of cargo as per the BL/ Charterer's instructions while ensuring the Drafts/GM/Stresses are within safe zone parameters.