I have a dropdown list and i want it to show the selected value on. the submit button – WordPress

Example of that behavior with jquery.
You need to match the select id
and also insert a span tag inside the button with a matching class

Live example:
https://jsfiddle.net/1qjvtc86/

From gravityforms site:

there is an easy way to add custom Javascript to your Gravity Forms
that only loads when the form is rendered. Just copy-and-paste your
desired snippet into the robust editor on your Form Settings and
you’re golden.

<div>
    <select id="picker">
        <option value="5">5 items</option>
        <option value="4">4 items</option>
        <option value="3">3 items</option>
        <option value="2">2 items</option>
        <option value="1">1 items</option>
    </select>
</div>
<div>
    <button>user picked <span class="pickedAmount">1</span> items</button>
</div>


<!-- copy this into gravity form -->
<script>
// picker - is the ID of the select
// pickedAmount - is the CLASS of the span 
jQuery(function($){
 
    $('#picker').on('change', function() {
        $('.pickedAmount').text($(this).val());
    })

});
</script>
<!-- copy this into gravity form -->