Okay, so it's been over a week since I made my first sandwich bread with guar gum instead of xanthan gum.
For the first and second loaves, I subbed guar gum for xanthan gum 1:1, and they failed miserably. For a recap, see here. Basically, the loaves were dry and crumbled into bits everywhere. I fed most to the birds, dried out some for breadcrumbs and salvaged the rest as french toast.
For the third loaf, I took some kind advice and subbed guar gum at ratio of 50% more than xanthan gum...so instead of 2 tsp, I used three. There was slight improvement - the loaf rose a bit more, but it still wasn't great. It tended to fall apart easily and the texture was off.
The fourth and last loaf is below.
You can see that the same problems are there - flat loaf, too wet, tendency to crack. For this last loaf I used 4 tsp. guar gum instead of 2 tsp. xanthan gum. At this rate, I'm not saving any money by using guar gum, which was one of my motivations. The end result is not terrible, but it's not the same as using xanthan gum. Again, adding more flour or using less water is not the answer. The batter is wet and the consistency is gummy no matter what I do. The loaf smells good and slices well, but it does not keep well. It makes good grilled cheese. But I'm used to better and I'm frustrated by this result.
In the end, if I had to make a loaf of gluten free yeast bread using guar gum instead of xanthan gum, I'd use twice the amount of guar gum than the xanthan gum called for in the recipe. But there will be differences. The girls are happy with it, so in the end it's okay. In everything else, subbing guar gum for xanthan gum works fine. For yeast bread there is a big difference. It's okay if you use the bread within a day or not two days after baking. After that, plan on making bread crumbs with the leftovers.