jekyll serve failing with Bundler::GemNotFound

Trying to start a webserver on localhost to check if my translation* works fine, but getting this:

mika@mika-desktop:~/minetest_modding_book$ jekyll serve
/home/mika/gems/gems/bundler-2.3.25/lib/bundler/definition.rb:507:in `materialize': Could not find jekyll-4.2.0, addressable-2.7.0, em-websocket-0.5.2, i18n-1.8.10, jekyll-sass-converter-2.1.0, kramdown-2.3.1, terminal-table-2.0.0, public_suffix-4.0.6, http_parser.rb-0.6.0, concurrent-ruby-1.1.8, listen-3.5.1, unicode-display_width-1.7.0, ffi-1.15.0, rb-fsevent-0.11.0 in locally installed gems (Bundler::GemNotFound)
	from /home/mika/gems/gems/bundler-2.3.25/lib/bundler/definition.rb:187:in `specs'
	from /home/mika/gems/gems/bundler-2.3.25/lib/bundler/definition.rb:244:in `specs_for'
	from /home/mika/gems/gems/bundler-2.3.25/lib/bundler/runtime.rb:18:in `setup'
	from /home/mika/gems/gems/bundler-2.3.25/lib/bundler.rb:161:in `setup'
	from /home/mika/gems/gems/jekyll-4.3.1/lib/jekyll/plugin_manager.rb:52:in `require_from_bundler'
	from /home/mika/gems/gems/jekyll-4.3.1/exe/jekyll:11:in `<top (required)>'
	from /home/mika/gems/bin/jekyll:25:in `load'
	from /home/mika/gems/bin/jekyll:25:in `<main>'

Did everything as in README. Seems like Minetest Modding Book needs older versions of these gems and refuses to serve.

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*So, yes, the code was modified, but only to add another language