so i dont have a full answer for you but you can use something called Accumulators which you will find under the ESM properties, custom types.
If you have custom fields that pull numeric data from a source, accumulator indexing can perform sums or averages over time on this data.
You can accumulate several events together and average their value or generate a trending value.
i dont think you can convert it to GB as such but you can accumulate by bytes. besides that i will see if i can find more information on how to set them up.