Solve error fetching gems from rubygems.org

The error was:

Could not fetch specs from https://rubygems.org/ due to underlying error <Net::OpenTimeout: execution expired (https://rubygems.org/specs.4.8.gz)

The solution was dissable IPv6:

sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1

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