Note, I’ve tested this in my testing environment, but please test this yourself before using on your website. but the following should work:
DELETE FROM gf_entry_meta;
DELETE FROM gf_entry;
DELETE FROM gf_entry_notes;
You’ll need to add your wordpress table prefix (found in wp-config.php) to the beginning of each table name.
This should remove all rows within the table but will not delete any columns nor the table itself.