I have used the West Systems 105 many times, but never tried the G Flex. It can be difficult at times to get a close fitting joint, but it is most important to try to achieve that goal. Gaps and voids are the enemy of a strong glue joint that won't break again in the future, so keeping and cleaning debris out of a crack or break is a priority, as is trying to find and keep all broken pieces of a stock that may be repaired. The original broken pieces will often fit better than any patch you might attempt, and the grain and pores will be perfect too.
Like you, I still have a quantity of the Brownells Acraglas dye for the times I do have to fill a void. Most often, that is going to be a natural void from an inclusion or internal crack in the walnut that cannot be pulled closed and glued. The color of the dyed epoxy has to approximate the color of the wood AFTER the finish is applied, because cured epoxy will not take stain or color deeper from the finish. So it may be necessary to put some finish on a hidden part of the stock, or a scrap from the same blank, to see what color you need. Many people advise coloring the clear epoxy with walnut sawdust, but that typically turns out too dark. I have found that mixing in some very fine sanding dust from pine or poplar will provide a better color match to the finished walnut. Adding a small amount of walnut sawdust may be needed if the mix is too light. Every piece is different, so it may take some experimentation to get a match that isn't glaringly obvious in the finished project. It takes a lot more time to do it right, but the results are worth it when you finally do it well enough that even you have a hard time finding a repair that you did.
