Question answered for Ubuntu but not working for Debian because it's not available yet. When searching packages.debian.org only packages for php 5 gd are available.
What's the preferred way of installing php7.0-gd in Debian 8 Jessie? What's the best unofficial source?
dotdeb.org appears to have it, install with instructions. And run apt-get update and apt-get install -y php7.0-gd.
It is available on version Stretch. This is the best official repo.
php7.0-gd: Installed: (none) Candidate:
7.0.12-1 Version table:
7.0.13-2 350
350 http://ftp.fi.debian.org/debian sid/main i386 Packages
7.0.12-1 500
500 http://httpredir.debian.org/debian stretch/main i386 Packages
echo 'deb http://httpredir.debian.org/debian stretch main' >> /etc/apt/sources.list.d/stretch.list