The Linux kernel. Beyond that, there are numerous alternative implementations of virtually every Linux program.
the kernal is the central part of the Linux operating system and determines how the system works - all distributions of Linux are based on this.
GCC is already for the most part preinstalled in many, if not all, GNU/Linux distributions.
The operating system, generally abbreviated as "OS". This could be Windows, Mac OS, Linux (any of dozens of different distributions or "distros" of Linux), Android, iOS, or many others.
For one: The Linux kernel. Otherwise it wouldn't be a Linux distribution. That's about it, actually. However, extremely common components include the GNU toolchain including glibc, desktop distributions will invariably deploy the Xorg server on top of which a desktop environment or window manager will run. As far as init systems go, System V is by far the most common, however systemd is starting to replace it.
SSH1 can technically be used on virtually all distributions, yes. It is not, however, in wide use due to certain security vulnerabilities.
For desktop computers, Canonical's Ubuntu, Linux Mint and Fedora are all very strong distributors for the operating system while for laptops, Fuduntu is a well known and good distributor for Linux.
Different Linux distributions can use different GUI's some will use Gnome or KDE. Most distributions will let you choose which you want. If you have an older machine you might want to use XFCE or LXDE window managers. If you are looking for a distribution you might want to try Linux Mint it uses all of these Window Managers.
Device support is reliant on the kernel. Depending on the kernel version in use and the policy regarding supporting experimental code and / or legacy components, one distro may support a device while another does not. but in general, all major Linux distributions for a platform have the same device support.
All versions of the Linux kernel since 2.0 support IPv6. Most modern distros have IPv6 enabled, but as there are over 300 different distributions, it is impractical or impossible to list all of them.
There are so many different ways they can differ I can't really cover them all. Virtually anything in Linux distributions can be replaced. Even the kernel can be swapped out with alternate builds. Usually, the most common changes are the default desktop and desktop applications for desktop distributions.
"Types" of Linux are Called Linux Distributions. Linux by itself is only a kernel, you need more than that for a full system, to get this, distro's were develpoed to include all of this to make a Linux system easier to install. Popular Distro's Are Ubuntu, Debian, Fedora, RHEL, Linux Mint, Arch Linux, OpenSUSE, etc, you can find more at distrowatch
Most Linux distributions including CD and USB do support wireless adapters. The problem is that not all adapters are supported.