This is based on a Elixir Phoenix application, but should work for elixir-only apps as well (they will have fewer directories, that's all).
A typical project would have these directories and files:
_build
assets
css
js
node_modules
package-lock.json
package.json
tailwind.config.js
vendor
config
config.exs
dev.exs
prod.exs
runtime.exs
test.exs
deps
lib
mix.exs
mix.lock
priv
test
For such a project I would probably add these to the ignore file:
_build
assets/node_modules
priv
# deps <- if the context is too large
With Gemini 2 Pro, and paid plan, this should be less of a problem