Octave - How to install packages on Windows
Question
Due to the issue in Fix for Octave urlread causing Peer certificate cannot be authenticated with given CA certificates, I cannot install Octave packages on Windows.
Please suggest other ways to install. Particularly I would like to go through Gradients, Gradient Plots and Tangent Planes which requires Symbolic package.
Answer
EDIT: this bug is no longer present in Octave v4.2.1, and the issue described in the Question should no longer occur.
Yes, there appears to be a known issue logged on the bug tracker with the current release version of Octave (4.2.0) on windows being unable to connect to https due to the curl issue you identified in the linked discussions/questions. That bug report and the original help list discussion summarize the certificate issue and problem verification. It should be fixed in the next Octave release.
This, however, does not prevent you from installing packages. It only prevents you from using the program to go fetch packages to be installed. You are still able to go to the Octave Forge package site, manually download a package file, and then as described in the Octave manual and help for pkg run the install command.
E.g., you could download symbolic-2.4.0.tar.gz and save it to your current working directory. Then within octave, issue the following from the command line:
pkg install symbolic-2.4.0.tar.gz
NOTE: symbolic currently requires Python and Sympy installed. If you don't already have this on your Windows machine, the package maintainer has a separate self-contained package for Windows that can be obtained from the author's github repository. In this case you would download the package and run the command:
pkg install symbolic-win-py-bundle-2.4.0.zip
Another more tedious option would be for you to compile your own copy from development sources, as the fix has supposedly been pushed to the mxe-octave repository.