Image Builders are platform-speicifc (ie: you need to download the image builder for the platform of your device, you cannot grab a random image builder and use it to create an image for the device of a different platform) and use profiles to target specific devices.
Maybe my terminology was wrong. I was referring to the information provided in this guide.
As stated it seems to have most target platforms but not the GL.inet device (MV1000) I was after. I am aware they have their own image building software but I want to run vanilla openWRT rather than their modified version.