Being used to GitHub Desktop on Mac, I found that SmartGit was the closest thing, and IMHO the best GUI client for Linux. It supports GitHub beautifully.
You have to unpack the download and run a script manually, but there’s also a script called “add-menu-item.sh” which will do what it says.
Requires Java (if you don’t have it, install with the following):
sudo apt-get install default-jre
Skype for Linux used to absolutely suck. Luckily, there is an alpha version of a new tool: https://www.skype.com/en/download-skype/skype-for-linux/.
It looks like a wrapper of Skype for Web, but it works.
For Whatsapp, there’s the now-adbandoned Whatsie. I think it was abandoned because there are now official clients for Windows and Mac, but it works perfectly.
Install .deb from here: https://github.com/Aluxian/Whatsie/releases/tag/v2.1.0.