My checking account shows a value that is higher then it actually is according to my bank statement. As far as I know I've only added transactions that have already been processed, so it should be the same as the value on my bank statement. I want to try to avoid having to go through all the real transactions to see which ones I missed, but also to correct the value in Gnucash. How should I do this? Maybe add an "Expenses - unknown" account? Or is there a more standard way of resolving this?
In general you need to ask yourself how serious you are about tracking your finances. If your GNUCash 'cleared' balance doesn't match your statement, it represents an error on either your part or much less likely the banks part. Tracking down this error might be a real pain, but you will also likely learn from it. So to answer your question - find the entry or entries that don't match and fix them.
That said, sometimes indeed this can be very tedious, time consuming and frustrating, especially if it is for a relatively small dollar amount. Time too is money, so in these cases, the 'Expense:Adjustment' might be a reasonable approach.
There is no "standard" way for personal accounting.
However, GNUCash default accounts set includes "Expense: Adjustment".
It is usually used by the community for reconciliation of unknown small money lost.