With a 3rd party Minecraft Launcher (Such as GDLauncher), do you still have to buy Minecraft Java Edition from
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.
Yes. While I very much know that you could easily find supposed "launchers" that are actually full versions of the game without a login and free, that would piracy and illegal.
Any (legal) launchers, clients, etc. will require you to pay for Minecraft.
If anything claims to run Minecraft without a Minecraft purchase it is illegal and also likely comes with malware.