For help:
./gnc_balance_assertion.py --help
To run assertions for a GnuCash file located at /path/to/account.gnucash, and where assertions are written as transaction descriptions of the form Balance Assertion: 123.45, you would run the following:
./gnc_balance_assertion.py path/to/account.gnucash --assertion_amount_regex "(?<=Balance Assertion: )[\-]*\d*\.\d*"
If you also want to reconcile accumulated amounts since a specific date, you might want balance assertions that look like this: Balance Assertion: 123.45 (since 2019-07-01):
./gnc_balance_assertion.py path/to/account.gnucash --assertion_amount_regex "(?<=Balance Assertion: )[\-]*\d*\.\d*" --assertion_start_regex "(?<=\(since: )\d\d\d\d-\d\d-\d\d(?=\))"
Note that any date format can be used as long as it is natively recognised by pandas.to_datetime()