The Go Keyboard AMP uses its own fonts created by the developers or by the fans. These fonts are typically found in the Play Store and you will have to install them through the Play Store. The custom fonts you add to your phone or install through other applications won't be available in the Go Keyboard AMP because this is the way the software has been created.
I've tried to use custom fonts and it didn't work. I had to install them through Play Store in order to appear in the font selection area.