/usr/share/doc/apt/NEWS.Debian says:
and their default settings (same on Debian buster and Ubuntu 20.04):
~$ apt-config dump | grep Cost
APT::Compressor::.::Cost "0";
APT::Compressor::zstd::Cost "60";
APT::Compressor::lz4::Cost "50";
APT::Compressor::gzip::Cost "100";
APT::Compressor::xz::Cost "200";
APT::Compressor::bzip2::Cost "300";
APT::Compressor::lzma::Cost "400";
So, basically setting any number under 50 to gzip's Cost will make apt use gzip to compress its indexes.
Anyway, I've send a PR, though I'm not sure its proven to work in ubuntu variants (not interested in their modiifications), and also "10" is good choice or not.
cf. https://github.com/armbian/build/pull/2729
