1/2" :) This is the measurement on the tree itself. I have recently learned that gullet is less important if the the flare of the bars is wider. Just bought a new saddle with a 6 3/4" gullet but the bars flare out wide. My horse is 15.3 and very stoutly built. It's too wide for him because of the flare of the bars. Now I am saddle shopping. So be sure to ask about that when you are looking.
Depends on the flare is. We have a Cactus Brandi Halls with FQHB and a Cactus Charmayne with FQHB. The Brandi saddle has straight bars and it doesn't fit anything we have right now, way too narrow. The CJ's fit everything perfect.