גדליהו: is a name; Gdalyahu - not an adjective.
כ"ו תשרי: is the date of his death, the letter כ stands for 20, the letter ו stands for 6.
נפט: comes from the word נפטר = deceased
As for the letters preceding Samuel Farkas's name: the correct way of abbreviating "our teacher the rabbi" is מו"ה and not what appears on the stone.
An error by the engraver? Or possibly, by the person who instructed the engraver?
Further research may clear this up for you.