If you are booting in EUFI mode you need to install grub in UEFI mode.
GRUB is the bootloader and efibootmgr is used by the GRUB installation script to write boot entries for efi systems.
# pacman -S grub efibootmgr
Mount the efi file system partition, for example it is /dev/sda2
# mount /dev/sda2 /boot
# grub-install --target=x86_64-efi --efi-directory=/boot --bootloader-id=GRUB
If you are having Windows as dual boot you ll need to add windows in grub boot menu, for that you need to install os-prober
# pacman -S os-prober
Mount the file system where windows is installed, for example windows 10 is installed on /dev/sda3
# mkdir /mnt/windows10 # mount /dev/sda3 /mnt/windows10
Generate grub configuration file
# grub-mkconfig -o /boot/grub/grub.cfg
Reboot system and you ll see grub bootloader with linux and windows boot entries.