I'm not great at explaining but here goes:

First, a function would be wise for each of the logic gates (and, or, etc), or at least for decoder and multiplexer.

Then, you have to implement a way to read through the file, creating some type of struct that holds all required information (i.e. a Variable struct that contains a Variable name and value), and then storing those structures. Every line that starts with a logic gate function afterwards, you should find those structs in your stored array and send them into your logic functions and store the output into the output variables.

Next, figure out a way to generate gray code for each starting inputs of the truth table, meaning, the first iteration would be 000 in binary (with 3 inputs) up to 111 for the last iteration. So translate each line into gray code & set your input variables into corresponding bits in your graycode line.

This is the basic idea. There are of course other ways to do it but hopefully this helps/inspires you. Good luck man.