sudo su
apt install apt-transport-https
curl https://apt.homegear.eu/Release.key | sudo apt-key add -
echo 'deb https://apt.homegear.eu/Raspbian/ stretch/' > /etc/apt/sources.list.d/homegear.list
apt update
apt install homegear homegear-homematicbidcos homegear-homematicwired
…worked for me. Thank you