You can add getData method in your js file:
getData: function() { return { 'method': "methodcode", 'additional_data': { 'customfield1': $('#'+this.getCode()+'_customfield1').val(), 'customfield2': $('#'+this.getCode()+'_customfield2').val(), } }; },
then you can get these fields in additional data param in your payment model:
namespace Namespace\ModuleName\Model; class Payment extends \Magento\Payment\Model\Method\Cc { public function assignData() { $post = $data->getData()['additional_data']; // write your custom code } }