You need basic knowledge of how to code with PHP (or know someone who can), you can also learn it at Codecademy.
Create a Chaingateway.io account (if you don't have one already).
Install the PHP 7 modules JSON and CURL. If you are on a Linux Debian/Ubuntu machine, you can use the following commands: apt-get install php7-curl apt-get install php7-json
Depending on your machine and PHP version, you might try the following instead: apt-get install php-curl apt-get install php-json
Replace YOURAPIKEY in the above code with the API Key in your Account panel.
Replace the other variables in the upper section of the code above with your data
Upload the created PHP script to your web server
Call the script in your browser. For example, if you upload it to the root directory of your website, you can call it by typing yourwebsite.com/sendtoken.php
After a few seconds, the script will return a transaction id (which means the transaction has been sent successfully). You can lookup this transaction id on Etherscan
Congratulations! You just sent ERC20 Tokens using Tokengateway. Wasn't that hard, right?
Want to know how to check the Token balance of an ethereum address? Check out this Tutorial