That's kind of what I was thinking, but I didn't autostrut anything. This gantry was giving me trouble on a much more complicated craft, so this was just a simple test to try and figure out what was happening. The other thought I had is that it might be tweakscale causing some issues, but I have the same problem whether the gantry is normal size, or scaled up. Here's a link to the craft file https://kerbalx.com/daspete/gantryTest