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For multiple Category selection options, you can add these checkbox options in one Gravity Forms field.
Checkboses and Radio Fields
When a checkbox is added as a custom field in SolarPlus, that custom field has an ID which can be seen in the URL when in the update mode for that field.
Use this ID as the 'value' field in your Gravity Form checkbox or radio widget.
When a Radio field is added in SolarPlus, each radio option has a value. Give the value integers such as 0, 1, 2...
In Gravity Forms, add an option with the Value matching the value field in the SolarPlus radio field.