I haven't done lemon brownies, but for the basic form, yes to 1:1 GF baking mix. Buckwheat flour is GF and also works 1:1, has great body (opposite of rice flour) but bakes gray, and might be hard to make appealing with lemon. A bit of buckwheat flour, however, might help with the texture. I love baking with almond flour, and it goes well with citrus, but you need a lot of eggs to make it light. Kind of opposite to brownies. But without the lightness I'm wondering if it'll be more gritty than fudgy texture. Try running it through the FP with sugar for the smoothest texture.

I make gluten free lemon bars frequently for family members that are gluten sensitive. They are just a layered version (more or less) of the brownie recipe. I use a GF 1-1 flour blend (Bob's Red Mill) for simplicity's sake and they turn out perfectly.

I find the 1-1 flour blend to be pretty much ideal for almost all of my GF baking requirements.

I have used King Arthur's GF (measure for measure?) and have that on hand. I've found it pretty satisfactory with cookies and muffins. I was just thinking that almond flour might help keep them moist and add some flavor. We are going to try a half batch for the meeting at the end of the month and then I can decide on any changes for another event in the fall. These teachers are such an appreciative audience.

I missed seeing the lemon brownie recipe, was it posted on another thread?

This isn't the thread where I saw them mentioned nor is it the one where I found the recipe. Even Google isn't helping me find those today, but I did find this thread where they show up as the first recipe posted by carol in california. They are quick and easy to make, but the one lemon and half a lemon frustrate me. My lemon had about half the juice I would expect to be a recipe average, so I adjusted accordingly. I plan to revise the copy of the recipe I keep to measure at least the juice -- peel varies a lot by lemon size also. But they are good, probably best made the night before - my son called them flat lemon cake and we all agreed they'd be called lemon bars or squares if that name didn't have an expectation of lemon curd or similar filling. Orange, grapefruit and possibly lime variations will be tried.

https://www.houzz.com/discussions/2565902/looking-for-fresh-lemons