Current up to date stretch (at date of this post):
aptitude install gcc-aarch64-linux-gnu
The following NEW packages will be installed:
0 packages upgraded, 1 newly installed, 0 to remove and 6 not upgraded.
Need to get 0 B/2,958 B of archives. After unpacking 19.5 kB will be used.
The following packages have unmet dependencies:
gcc-aarch64-linux-gnu : Depends: gcc-5-aarch64-linux-gnu (>= 5.2.1-13cross1) which is a virtual package and is not provided by any available package
The following actions will resolve these dependencies:
Keep the following packages at their current version:
1) gcc-aarch64-linux-gnu [Not Installed]
Accept this solution? [Y/n/q/?]