The sad news is that it technical not possible to run existing c source code on a forth cpu. The reason is, that Forth cpus have no registers but only a minimal stack. And no c compiler in the world is able to generate the assembly instructions for such a machine. Even for mainstream CPUs like the 6510 it is hard to write c compiler because this CPU has also a low amount of registers. And in a forth cpu the situation is more dramatically.
So the only practical method to write software for the GA144 and other CPU is to hand code the software in Forth. Unfortunately, Forth is known as one of the hardest programming languages ever next to Dyalog APL. An easy way to learn the Forth language should be explained in the following blog post.
Instead of trying to understand the idea of a push down stack the more beginner friendly approach to write something in Forth is to use modular programming. Modular programming is a powerful software engineering method which is supported by most languages like Pascal, C, Python, C++ and of course Forth. The idea is that each file contains of 4-5 functions plus a handful of variables. The functions can access the variables and the module can be included by other modules. The concept is some sort of strip down version of a class.
Modular programming in Forth is a bit more complicated than doing the same in C. but it is not impossible in writing such programs. It is mostly a question of getting experience in writing stack based functions. The concept of modular programming is the same. It allows that the source code can grow to a size of 1000 lines of code and even more. And each single file has a limited size of not more than 100 lines of code. If the file is getting bigger, the programmer has to outsource some of the routines and creates some interfaces to communicate with other modules.
The interesting situation is that Forth based modular programming works well with existing Forth cpus. The only thing what a certain Forth system needs to support is the ability to execute a word, and also the ability to read and write to variables.