Only Enable the Last Day of Each Month

<?php
/**
 * Gravity Perks // Limit Dates // Only Enable the Last Day of Each Month
 * https://gravitywiz.com/documentation/gravity-forms-limit-dates/
 */
// Update "123" to your form ID and "4" to your field ID.
add_filter( 'gpld_limit_dates_options_123_4', function( $options, $form, $field ) {

	$range_start = new DateTime();
	if ( (int) $range_start->format( 'j' ) > 1 ) {
		$range_start->modify( 'last day of this month' );
	}

	$cloned_date_range = clone $range_start;
	$range_end         = $cloned_date_range->add( new DateInterval( 'P1Y' ) );

	$period = new DatePeriod( $range_start, new DateInterval( 'P1M' ), $range_end );

	if ( empty( $options['exceptions'] ) || ! is_array( $options['exceptions'] ) ) {
		$options['exceptions'] = array();
	}

	foreach ( $period as $date ) {
		$options['exceptions'][] = gmdate( 'm/d/Y', strtotime( 'last day ' . $date->format( 'm/01/Y' ) ) );
	}

	$options['disableAll']    = true;
	$options['exceptionMode'] = 'enable';
	$options['minDate']       = $range_start->format( 'm/d/Y' );
	$options['maxDate']       = $range_end->format( 'm/d/Y' );

	return $options;
}, 10, 3 );