Cannot pass GF checkbox values to ECPT fields

When we try to pass multiple checkbox values to any fields in ECPT, including textarea fields, only the first variable is succesfully submitted. Is there a way to have a checkbox form element in the front end, but which submits a comma separated text list?

No special method, just choosing the custom field to populate in gravity forms. So I have a checkbox in the form with a number of possible values, which populates a custom field in a custom post type, both custom fields and CPT's have been created in ECPT plugin. The selected values in the form are not coming into the custom field, just the first checkbox value, nothing else. Have tried creating the custom field as text field, or also a checkbox but it doesn't make any difference, still the same result. I have talked to the ECPT creator and he says there must be an incompatibility between the way GF passes the field values into ECPT CF's. Anyone heard of this, or some way to work around it?