The gem would have to have a js package that can be installed with yarn/npm so that it can be imported in application.js. If it doesn't, you can setup a js file to be processed only by sprockets, like in the old days.
Add another javascript entry point that will skip esbuild and will be processed only by sprockets.
Update manifest:
// app/assets/config/manifest.js
//= link custom.js
Add //= require
directive:
// app/assets/javascripts/custom.js
//= require gem_javascript
Add it to layout:
<!-- app/views/layouts/application.html.erb -->
<%= javascript_include_tag "application", "custom", "data-turbo-track": "reload", defer: true %>
Alternatively, instead of using //= require add gem_javascript to javascript_include_tag
:
<%= javascript_include_tag "application", "gem_javascript", "data-turbo-track": "reload", defer: true %>
Might have to add it to manifest as well for precompilation:
// app/assets/config/manifest.js
//= link gem_javascript