Perhaps they are in a different set of plugins from Guitarix. You can try the following:
ssh root@zynthian.local
cd /zynthian
git clone https://github.com/dcoredump/zynthian-recipe.git
cd zynthian-recipe/recipe
sh guitarix-new.sh
This installs some additional plugins for guitarists (After installation you have to restart MOD-UI!).
Regards, Holger