I admire your efforts to read the Hebrew.  Tombstones are particularly challenging, as they are written without vowels, phrases run over from one line to the next, there is no punctuation, and there are many abbreviations.

Here is a full transliteration:

Lezichron Nishmat

Avinu Hayakar

Mem-Hay-Vav (as Ms. Zlotnick explained, probably a 'typo', and should be Mem-Vav-Hay, an abbreviation that represents the words Moreinu HaRav)  Shmuel

Bar (an abbreviation for ben Reb) Gedalya haCohen

Nift[ar] chav-vav Tishri Taf-Resh-Pay-Vet

Taf-Nun-Tzadi-Bet-Hay [= Tehay Nishmato Tzrurah Bitzror Hachayim]

=========================================================
This translates to:

As a remembrance for the soul of

our dear father

our teacher, the rabbi, son of Reb Shmuel Gedalya the Cohen

He passed away 26 Tishri [5]682

May his soul be bound up in the bond of life.

As Ms. Zlotnick pointed out, Hebrew letters have numerical value.  On this tombstone, this can be seen in the date, where chav-vav equal 26.  It can also be seen in the year, where taf=400, resh=200,fay=80,vet=2, for a total of 682 (the leading 5 is understood and does not appear).
The 26th of Tishri of the year 5682 began at sunset on October 27th, 1921, and ended at sunset on the 28th.

The abbreviation "our teacher, the rabbi" normally indicates that the person had rabbinical training (not necessarily that he was the rabbi of a congregation).  I have found that this abbreviation is often used in error, so I'd suggest taking this with a grain of salt, unless you have another source to confirm his rabbinical status.

The 'Reb' in front of his father's name is an honorific; it does not mean rabbi.

By the way, if you have other gravestones to interpret, I recommend posting them to the ViewMate feature of jewishgen, where images can be uploaded and there is a specific "translation-Tombstone" category (https://www.jewishgen.org/ViewMate/).

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Fredel Fruhman
Brooklyn, New York, USA