If the payment gateway assembly is not found by Kentico CMS, your custom payment gateway form will not be loaded.
Following properties of your custom payment gateway are case sensitive, so you need to be sure that you specify them correctly (according to Developing custom gateways) :
Payment gateway assembly name:
Payment gateway class name:
You also need to ensure that you are using correct dll files with your custom payment gateway dll file. In other words, if you have built your dll file in your development environment and you are delivering the custom payment gateway dll file to your live web site, please copy all of the other Kentico CMS dll files together.
Also please check Kentico CMS Event log (CMS Site Manager -> Administration -> Event log) for possible error messages which might lead you to the cause of this issue.