I would like to add default answers to my custom tables, but cannot seem to be able to find how it is done. Is this possible?
Unfortunately it is not possible to set default answers in custom tables.
You can contact SurveyGizmo Support for a more detailed answer (https://community.surveygizmo.com/email-a-support-hero/), but basically each row of a table-type question is treated as a separate question. Each row of a custom table is treated as a series of sub-questions. This becomes more apparent when you look at how the responses to a custom table are reported (https://help.surveygizmo.com/help/custom-table#reporting).
Depending on your needs, you might want to consider using a Custom Group question instead (https://help.surveygizmo.com/help/custom-group). You can use the LINE BREAK function found under the LAYOUT tab to place questions in a row.