f1.rb

# ц
puts `chcp`
p Encoding.default_external
p file:__FILE__
p dir:__dir__

puts '-'*10
p LOAD_PATH:$LOAD_PATH[0]
p start_with:$LOAD_PATH[0].start_with?(__dir__)

# ----- workaround --------------------------------------------------------
# p Gem_dir:Gem.dir
# 	Gem.dir.force_encoding("UTF-8")
# 	p Gem_dir:Gem.dir
# p Gem_path:Gem.path
# 	Gem.path.each {|path| path.force_encoding 'UTF-8' }
# 	p Gem_path:Gem.path

# if $:[0].encoding.name == 'Windows-1251'
#     $:.each {|path| path.encode! 'UTF-8' }
#     $:.push '.'    # somehow it helps, looks like a modification of array is needed
# end
# ----- /workaround --------------------------------------------------------

# puts 'bundler/setup'
# # fails: incompatible character encodings: ASCII-8BIT and UTF-8 (Encoding::CompatibilityError)
# require 'bundler/setup'

puts '='*10
require_relative 'f2'

f2.rb

# ц
puts `chcp`
p Encoding.default_external
p file:__FILE__
p dir:__dir__

puts '-'*10
p LOAD_PATH:$LOAD_PATH[0]
# fails: incompatible character encodings: Windows-1251 and UTF-8 (Encoding::CompatibilityError)
p start_with:$LOAD_PATH[0].start_with?(__dir__)
# fails: incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)
p start_with:$LOAD_PATH[0].start_with?(Gem.dir)

puts 'bundler/setup'
# fails: incompatible character encodings: ASCII-8BIT and UTF-8 (Encoding::CompatibilityError)
p require 'bundler/setup'