This library will allow you to populate a series of fields in 2-week increments.
https://gravitywiz.com/populate-dates-gravity-form-fields/
The configuration would look something like this:
new GW_Populate_Date( array(
'form_id' => 123,
'source_field_id' => 1,
'target_field_id' => 4,
'modifier' => '+2 weeks'
) );
new GW_Populate_Date( array(
'form_id' => 123,
'source_field_id' => 1,
'target_field_id' => 5,
'modifier' => '+4 weeks'
) );
new GW_Populate_Date( array(
'form_id' => 123,
'source_field_id' => 1,
'target_field_id' => 6,
'modifier' => '+6 weeks'
) );
The source field is the field that the user would select the date form. The target field would be populated with a new date based on the source date and the modifier.
In this example, we target a new target field with each instance and increment the modifier by 2 weeks. The source field and form ID stay the same.