readme.md

This script changes a text input field into one witha £ prefix and number only

Change the id or selector - the example shows #input_1_7 which is Gravity forms notation ( Form 1 field 7 )

Easiest way it to add an html block on the form page - although of course you can enqueue scripts etc

other ways

I'd love for someone to show me how to do this in pure JS without jQuery or ES6

gravitynum.js

<script>
(function ($) {
    'use strict';
    $(document).ready(function ($) {
        $("#input_1_7").blur(function () {
            $(this).val('£' + $(this).val().replace(/[^0-9]+/g, ''));
        });
    });

})(jQuery);
</script>