Magento 2: How to Get Guest Email Address in Checkout?

You can get guest and customer other information in your custom js like customerinfo.js in Magento_Checkout/js/model/quote:

define(
    [
        'jquery',
        'Magento_Checkout/js/view/payment/default',
        'Magento_Checkout/js/model/quote',
        'Magento_Checkout/js/action/place-order',
    ],
    function ($,Component,quote,placeOrderAction) {
        'use strict';

        return Component.extend({

            defaults: {
                template: 'Company_Module/custompayment/form_template'
            },

            getEmail: function () {
                if(quote.guestEmail) return quote.guestEmail;
                else return window.checkoutConfig.customerData.email;
            },

            getLastName: function () {
                return quote.billingAddress().lastname;
            },

            getFirstName: function () {
                return quote.billingAddress().firstname;
            },

            getAddress: function () {
                return quote.billingAddress().street[0]+", "+quote.billingAddress().postcode+" "+quote.billingAddress().city+", Maroc";
            },

            getPhone: function () {
                return quote.billingAddress().telephone;
            },

        });
    }
);


You can get guest's email id without login in checkout page, use avobe code.

Through this process you will able to gather customer's info which is improve your conversions.