Baked goods with Teff flour turn out gritty. How to prevent this

It depends on what you mean by gluten-free flour. If you are buying a gluten-free flour mix that it labeled for all-purpose use then you should be able to substitute that and get a reasonably good result. Substituting straight rice or almond flour in a standard recipe will not work well, because the mixes have a blend of ingredients designed to make it perform similar to flour.

It may take some recipe/flour experimentation to get it as close to what you are used to, but will definetly make something edible. There are also various almond-flour based pancake recipes which work well.

My step daughter also has celiacs/coeliacs disease. We often use all-purpose gluten free flour for soups, cheese sauce and roux etc. I can honestly say there is no noticeable difference. The only thing I would recommend is if the recipe asks you to make a roux don't try and cook out the flour as it will turn lumpy. Instead just melt the butter, add the flour mix it in then go straight in with the milk/cream (all of it, don't do a little at a time, as again it'll go lumpy) I'd stick to the same ratio (1:1) this time and then alter to your liking for next time.

You could by all means use just corn flour but if you want a thick soup and this is the only thickener you may end up with gloop instead. If you do use corn flour it's best to stir it in at the end, mix it with cold water then just add a table spoon of the mixture at a time until it's the right consistency.