Transferring Variable Information Between GUI and Projectile

One part of the mod I am attempting to create requires that a GUI will communicate a variable to a projectile when it is shot by the player. I have tried a few different ideas with little success and was wondering if anyone had any ideas to attempt or tutorials? I have tried searching the forums/youtube so far but haven't found anything yet. The end goal is to create a GUI that players can put items into that will change the projectile's behavior and that each player's GUI would be separate. However, even a single-player solution would be a great start.

I currently have enough ability to make basic codes for other parts of the mod and add ores/blocks/textures and am excited to learn more!