Add gift item to the cart above specific cart value. Ref: https://www.tychesoftwares.com/how-to-automatically-add-a-product-to-your-woocommerce-cart-in-3-different-scenarios/

add_gift_to_cart.php

<?php

/*
* Automatically adding the product to the cart when cart total amount reach to $500.
*/

function aapc_add_product_to_cart() {
	global $woocommerce;
	
	$cart_total	= 500;	
	$free_product_id = 111111;  // Product Id of the free product which will get added to cart

	if ( $woocommerce->cart->total >= $cart_total ) {
		if ( ! is_admin() ) {
	        $found 		= false;

	        //check if product already in cart
	        if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
	            foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
	                $_product = $values['data'];
	                if ( $_product->get_id() == $free_product_id )
	                	$found = true;	                
	            }
	            // if product not found, add it
	            if ( ! $found )
	                WC()->cart->add_to_cart( $free_product_id );
	        } else {
	            // if no products in cart, add it
	            WC()->cart->add_to_cart( $free_product_id );
	        }        
	    }
	} else {
		//To remove the Freebie in case user deletes all items from the cart
		foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
     		if ( $cart_item['product_id'] == $free_product_id ) {
          		WC()->cart->remove_cart_item( $cart_item_key );
     		}
		}
	}
}