The group by filter converts a array of transactions to an array of arrays of transactions, ie a group of arrays of transactions.
To display the results you have to cycle through both levels. The outer loop cycling through the group, the inner loop cycling through the transactions. For example
Unfortunately last time I tried after you use the group by filter, then the balance: and drill down no longer work, so have to be all calculated manually.
edit
group_by: GeneralLedgerAccount.Name
is probably closer to valid code.
{% for account in openingBalance %}
{% for f in account %}
{{ f.Date }} {f.Amount }}
{% endfor %}
{% endfor %}