Pascal researched and found that the error, anyconnect was not able to establish a connection to the specified secure gateway is a known problem with Cisco clients before version 4, when these earlier clients are installed on Ubuntu 16.04+. The solution is either to downgrade your Ubuntu, or upgrade your Cisco client. At my university we have upgraded to offering version 4 (anyconnect-predeploy-linux-64-4.3.05017-k9.tar.gz), and this supposedly works with the newer Ubuntus. I did not myself test the new version 4 Anyconnect client with Ubuntus 15.x and 16.x. But I have tested it today (April 27 2017) with my Ubuntu 17.04 system, and it works great.
The Cisco AnyConnect Secure Mobility Client web deployment package should be downloaded to the local desktop from which the ASDM access to the ASA is present. In order to download the client package, refer to the Cisco AnyConnect Secure Mobility Client web page. The web deployment packages for various Operating Systems (OSs) can be uploaded to the ASA at the same time. 1e1e36bf2d