Firstly, assuming that there was no Agent's commission, AND that no one-off ledger payments were made during a month, then the GST total for the month should be 15% or 10% of all the other fee amounts - to within about a dollar to allow for rounding.
However, if there were one-off ledger payments (e.g. to pay for a casual advert, bank fees, or to refund fees), then it is common for a user to take the money from a fee account (Management Fees) but not to also take some from the GST ledger. This would throw the GST balance to be no longer 15% of the total of other fees.
A further complication comes in with Agent Commissions.
As transactions are processed, part of the fees are separated out for each agent earning commission, BUT part of the GST will only be allocated to the Agent if his Agent record holds a GST number - otherwise he will only get the Fee amount and the GST portion will remain in the GST ledger.
Again, if no one-off payments have been made out of the ledger fee accounts, the total of all fee balances PLUS the total of agent commissions balances can be used to check the GST balance - assuming no agents were getting GST.
If some agents are being allocated GST money as well as commission, and some not, then it is difficult to determine the exact GST amount. However, right clicking the GST ledger and doing a details report will show all the GST, and at the end how much of it was allocated to Agents.
Because of the various possibilities, we suggest that you add all fee balances plus the GST balance plus all agents' money, then divide by 7.666666 to get the GST figure.