@Biddaum 

so your requirement is simply to show modal and 2 buttons

Proceed and Cancel

On click of Cancel take them to portal home?

On click of Proceed close the modal and let them submit form?

@Ankur Bawiskar yes that is correct

@Ankur Bawiskar 

works perfectly thank you

You have to set isolate script to false, then you can access the window object from this.

function onLoad() {
    //Type appropriate comment here, and begin script below

    if (typeof spModal !== 'undefined') {

        if (g_scratchpad.isFormValid) {
            return true;
        }

        var actionName = g_form.getActionName();

        spModal.open({
            backdrop: 'static',
            keyboard: false,
            message: 'Please select cancel if you want to go back',
            title: 'This is a test',
            buttons: [{
                    label: 'Cancel',
                    cancel: true
                },
                {
                    label: 'Proceed',
                    primary: true
                }
            ]
        }).then(
            function onAgree(result) {
                g_scratchpad.isFormValid = true;
                g_form.submit();
                return true;
            },
            function onDisagree(reason) {
                setTimeout(function() {
                    this.location.href = 'https://www.testing.com';
                }, 0);
                return false;
            }
        );

        g_scratchpad.isFormValid = false;
        return false;

    } else {
        var gm = new GlideModal();
        gm.setTitle('Error');
        gm.renderWithContent('Something has gone wrong Prompt');
    }
}

 To successfully submit with gform on the catalog item widget you also need to do something like this. Make sure you dont have two step checkout on the item

const $scope = angular.element("#sc_cat_item").scope()
$scope.data.sc_cat_item.item_action = "order";
g_form.submit()
//or maybe instead of submit use
$scope.triggerOnSubmit()