I second seeing a developmental optometrist. My ds fits the profile here too. He's great visual-spatial ability and is a whiz at math -- until he got to geometry too. He's got some visual spatial deficits and an issue with his visual working memory. I didn't realize how bad and uneven ds was with it until we saw the behavioral optometrist a couple of months ago. Then again, ds had a tbi and a blind spot too with his vision as a baby.
A developmental optometrist should be able to find any visual spatial deficits that otherwise might not be picked up. These things can be subtle and it's often wise to get a second opinion especially since about 80% or more of what we process involves the visual system. The issue is that gifted kids with visual spatial deficits can compensate so well that the deficits can go unnoticed until they hit a brick wall and spiral out of control.
Compensation is iffy. Some days it works; other days it doesn't. Moreover, you're often expending more energy than necessary. Vision therapy can make a difference. Behavioral optometrist told me that it's not uncommon for gifted kids to have their vsl abilities and bodies lag with their brains.
Geometry involves a considerable amount of terms, vocabulary, and abstract reasoning compared to algebra. This can also pose a problem with 2e or gifted kids. Perfectionists can panic and get overwhelmed by it. When things come effortlessly and then a child has to suddenly 'work' at it, they not understand why or what to do.
My suggestion is to make geometry as concrete as possible so it's less intimidating and seems more manageable. I took ds around the house and then outside looking for lines, line segments, rays, points, parallel lines, non-parallel lines; we also made a lot of jokes because humor can help with vsl memory.
There's some catchy songs on Know Watch Learn (
http://www.watchknowlearn.org/) which might help. Education portal relates angles to pizza and other food (
http://education-portal.com/) - that seems to be more appealing to my ds too.