Proposal for GravityForms to add a 'gform_custom_field_name_clauses' filter.

forms_model.php

<?php 
class GFFormsModel {

	public static function get_custom_field_names() {
		global $wpdb;

		$clauses = apply_filters( 'gform_custom_field_name_clauses', array(
			'found_rows' => '',
			'distinct'   => 'DISTINCT',
			'fields'     => 'meta_key',
			'join'       => '',
			'where'      => "AND meta_key NOT BETWEEN '_' AND '_z'",
			'groupby'    => '',
			'having'     => 'HAVING meta_key NOT LIKE %s',
			'orderby'    => 'ORDER BY meta_key',
			'order'      => 'ASC',
			'limit'      => ''
		));

		$sql = "SELECT {$clauses[ 'found_rows' ]} {$clauses[ 'distinct' ]} 
            {$clauses[ 'fields' ]}
			FROM $wpdb->postmeta {$clauses[ 'join' ]}
			WHERE 1=1 {$clauses[ 'where' ]}
			{$clauses[ 'groupby' ]}
			{$clauses[ 'having' ]}
			{$clauses[ 'orderby' ]} {$clauses[ 'order' ]}
			{$clauses[ 'limits' ]}";

		$keys = $wpdb->get_col( $wpdb->prepare( $sql, $wpdb->esc_like( '_' ) . '%' ) );

		return $keys;
	}
}