I answer 'no' because gdisk and cgdisk are already included in Slackware and in Slackware's installer.
Slackware's installer says it right after the (bogus) login:
To partition your hard drive(s), use 'cfdisk' or 'fdisk'.
For systems using GPT instead of MBR, partition using 'cgdisk' or 'gdisk'.
To activate PCMCIA/Cardbus devices needed for installation, type 'pcmcia'.
To start the main installation (after partitioning), type 'setup'.
EDIT Furthermore, gfdisk is an interface to libparted, which in turn is included in Slackware. So it *could* be useful to some. But many people who used to use (may I say that in English?) parted often use gparted instead nowadays.
PS I am *not* requesting that gparted be included in Slackware. I have a live DVD of it already