3rd party Minecraft Launcher
With a 3rd party Minecraft Launcher (Such as GDLauncher), do you still have to buy Minecraft Java Edition from minecraft.net?
Yes, and with your provided example (GDLauncher), the first thing required in order to launch any game instance (or to do even anything at that) is to log in with your Mojang account.
Other launchers such as MultiMC also require a Mojang login.
Keep in mind that some of these 3rd party launchers may need to change because of the Microsoft account migration, but I could be mistaken here.