gtbl and troff are part of the groff package. The groff package provides versions of troff, nroff, eqn, tbl, and other Unix text-formatting utilities. Groff is used to 'compile' man pages stored in groff/nroff format into a form which can be printed or displayed on the screen. These man pages are stored in compressed form in the /usr/man/ directories
Groff probably was not installed - that all.
Pull out your slackware disk and in the groff package, or goto (eeks - bad word
And see if you can find it there.