Is there any way (preferably free) to create your own GUI interface for your own sampler?
The popular .sfz file format player Sforzando allows sample library creators to make GUI. You would have to get in touch with the developer of Sforzando though:
Another great and fairly new option is a new open source framework for sampled instruments called HISE. It allows you to create and export sample instruments in it’s file format, but it also allows you to create custom sample based instruments with it’s own GUI. It’s intended as an open source alternative to Kontakt, including scripting: