For folks that try to use old GTS 450 or similar legacy-cards - the easiest way to get them working is by using some KDE editions of this distro. Simply install the nvidia-390xx-dkms from AUR. In case you're having any vsync or performance problems - https://wiki.archlinux.org/index.php/NVIDIA/Troubleshooting#Avoid_screen_tearing_in_KDE_(KWin) Also, I tested the Wayfire and i3 editions and they had similar issues with these cards - the WM just refuses to boot up, so can't recommend these.
Thanks for posting, but those are basically temporary workarounds. The only real solution is to not buy your graphics cards from a manufacturer that doesn't support open source drivers.
Nvidia has deprecated the drivers for my last 2 nvidia card purchases. I will never buy any of their products again as they are not Linux/open source friendly. Fool me once shame on you, fool me twice shame on me.
Also, every time when you need to change a kernel you need to rebuilt your driver for it. I usually just boot up from grub (Advanced options > Garuda with a new kernel), then do "Ctrl+Alt+F2" (Or any other F, except for the letter ) to change to a different display server and start building. In case of this distro, you can just log in and do "pacman -S nvidia-390xx-dkms" and reboot.