This will fix on a standard installation, if you do manual partition, ask at the forum!
Boot up with our LiveIso.
mount System to /mnt
You need to know which device is the right one check with
sudo fdisk -l
(/dev/sdXn should be replaced with the right device information)
mount /dev/sdXn /mnt >> / root partition
mount /dev/sdXn /mnt/boot >>> /boot partition
mount /dev/sdXn /mnt/boot/efi >>>>> the EFI-boot (fat) partition
BUT that grub is not in the boot list can be caused also by EFI-Boot-Settings not related to Grub and Antergos then, so re-/installing-/building grub may not solve your problem…
You can also make shure to get second (dual boot) installation rechecked and put into grub boot menu:
(as root also) If you have dualboot os-prober may takes a long long time to work, the trick here is to mount all other partitions, before using it.
os-prober grub-mkconfig -o /boot/grub/grub.cfg
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=Antergos-grub
For legacy (bios) systems go here: https://antergos.com/wiki/miscellaneous/how-to-fix-grub-with-bios-boot/