This function allows you to programatically create GravityForms coupon.

gravityforms-create-coupon-example.php

<?php

function zawp_create_coupon_codes() {
  if ( isset( $_GET['create_coupons'] ) ) {
    $coupons = array(
      array(
        'coupon_name' => 'Agnieszka K - OCC - SML client code 100€ OFF',
        'coupon_code' => 'AgnieszkaK100',
        'amount' => '100',
        'type' => 'flat',
        'entry' => '',
        'form'  >= 0,
      ),
    );

    foreach ( $coupons as $coupon ) {
      zawp_create_coupon_gf(
        $coupon['coupon_name'],
        $coupon['coupon_code'],
        $coupon['amount'],
        $coupon['type'],
        $coupon['entry'],
        $coupon['form'],
        1
      );
      echo 'created.... ' . $coupon['coupon_name'];
    }

  }
}
add_action( 'admin_init', 'zawp_create_coupon_codes' );

gravityforms-create-coupon.php

<?php

/**
 * Create Coupon Codes
 * 
 * This function allows you to programatically create GravityForms coupon. 
 *
 * @param string $coupon_name
 * @param string $coupon_code
 * @param integer/string $amount
 * @param string $type
 * @param string/integer $entry
 * @param integer $form
 * @param integer $limit
 * @return void
 */
function zawp_create_coupon_gf( $coupon_name, $coupon_code, $amount, $type, $entry, $form, $limit ) {

  if ( ! class_exists( 'GFCoupons' ) ) {
    return;
  }

  // hack to load GF Coupons data.php file
  if ( is_callable( 'gf_coupons' ) ) {
    gf_coupons()->get_config( array( 'id' => 0 ), false );
  } else {
    GFCoupons::get_config( array( 'id' => 0 ), false );
  }

  $meta = array(
    'form_id'           => false,
    'coupon_name'       => $coupon_name,
    'coupon_code'       => $coupon_code,
    'coupon_type'       => $type, // 'flat', 'percentage'
    'coupon_amount'     => $amount,
    'coupon_start'      => '', // MM/DD/YYYY
    'coupon_expiration' => '', // MM/DD/YYYY
    'coupon_limit'      => $limit,
    'coupon_stackable'  => false,
  );

  $form_id = $meta['form_id'] ? $meta['form_id'] : 0;
  unset( $meta['form_id'] );

  if ( is_callable( 'gf_coupons' ) ) {
    $meta['gravityForm']      = $form_id ? $form_id : 0;
    $meta['couponName']       = $meta['coupon_name'];
    $meta['couponCode']       = $meta['coupon_code'];
    $meta['couponAmountType'] = $meta['coupon_type'];
    $meta['couponAmount']     = $meta['coupon_amount'];
    $meta['startDate']        = $meta['coupon_start'];
    $meta['endDate']          = $meta['coupon_expiration'];
    $meta['usageLimit']       = $meta['coupon_limit'];
    $meta['isStackable']      = $meta['coupon_stackable'];
    $meta['usageCount']       = 0;
    gf_coupons()->insert_feed( $form_id, true, $meta );
  } else {
    GFCouponsData::update_feed( 0, $form_id, true, $meta );
  }

}