Use ruby-lsp plugins without modifying the project's Gemfile

First, avoid including editor-specific configuration files in the project.

# .gitignore OR .git/info/exclude
.vscode

Next, prepare a Gemfile for ruby-lsp in the gitignored directory. If .ruby-lsp/Gemfile already exists, you can copy and paste and edit it. In this example, I will introduce a new plugin called ruby-lsp-rspec.

# .vscode/Gemfile
eval_gemfile(
  File.expand_path(
    '../Gemfile',
    __dir__
  )
)

group :development do
  gem "ruby-lsp", require: false
  gem "ruby-lsp-rails", require: false
  gem "ruby-lsp-rspec", require: false
end

Finally, change the configuration of ruby-lsp to specify the path to the Gemfile. This may be set from the GUI.

// .vscode/settings.json
{
  "rubyLsp.bundleGemfile": ".vscode/Gemfile"
}

Reloading the workspace will reflect the settings; you can also use VSCode's reloadWindow command. Currently, ruby-lsp seems to have a bug that an error occurs if this setting is changed at startup, in which case you can reload it twice.