גדד

Abarim Publications' online Biblical Hebrew Dictionary

גדד גדה

The two forms גדד (gdd) and גדה (gdh) may be related etymology and their meanings appear to overlap somewhat:

The verb גדד (gadad), means to cut or invade but with a very special connotation.

According to HAW Theological Wordbook of the Old Testament, in nine out of eleven occurrences this verb specifically refers to forbidden self-laceration in an act of worship (Deuteronomy 14:1, 1 Kings 18:28, Jeremiah 16:6). And when in Genesis 49:19 Jacob blesses his sons on his deathbed, he says of Gad (whose name comes from this verb):

Jacob	Gad		Gad	Gad	Gad
עקב	יגד	והוא	יגדגו	גדוד	גד
their heel	shall cut	and he	shall raid them	raiders	Gad

This verb's derivatives, however, reveal the deeper meaning:

The masculine noun גדוד (gedud), meaning a band of raiders, possibly (as BDB Theological Dictionary suggests) in the sense of invaders or a guerilla group severed from a main group or society (1 Samuel 30:8, 1 Kings 11:24). In later Scriptures this word also came to denote military divisions in Israel (1 Chronicles 7:4, Micah 5:1).
The identical masculine noun גדוד (gedud), and its feminine counterpart גדודה (geduda) both meaning furrow, cutting (Psalm 65:10, Jeremiah 48:37).
The noun גד (gad), possibly meaning fortune (Genesis 30:11). In Isaiah 65:11 this word occurs as possibly the divine name Gad (see the translations of Young, Darby and JSP). The NIV, NAS and ASV versions interpret this occurrence of גד (gad) as a name but translate it with Fortune. KJV reads "that troop".

It seems that the verb indicates a cutting with the distinct purpose of laying a treasure bare. The ritualistic cutting results in the exposure of precious blood, and a raid produces loot. But note that the somewhat similar verb אגד ('agad) means to bind in the same sense that גדוד (gedud) denotes a band of raiders.

גדה

The verb גדה (gdh) isn't used in the Bible, but BDB Theological Dictionary proposes a meaning of to cut, cut or tear away. And that obviously links it to the verb גדד (gadad) treated above. In the Bible only the following derivatives remain:

The feminine noun גדה (gadda), meaning bank (of a river). It occurs only in Joshua 3:15, 4:18, 1 Chronicles 12:6 and Isaiah 8:7.
The masculine noun גדי (gedi), meaning kid (young animal), and that almost exclusively as source of a meal (Genesis 27:9, Judges 6:19, 1 Samuel 10:3).
The feminine plural noun גדיה (gediya), meaning kids (Song of Solomon 1:8 only).

It's a bit of a mystery how a young animal has anything to do with a river that cuts through a landscape, but perhaps the ancients saw the visual similarity between a large herd and a large body of water (especially herds of moving deer and the likes behave very much like a liquid). Or perhaps they saw the progression of a species as a river with branches, not unlike our modern evolutionary "tree". But on the other hand, in the olden days a person's wealth was measured according to the amount of cattle he possessed, and every kid was in fair fact a walking dollar, insinuating that the noun גדי (gedi) may have to do with the noun גד (gad) meaning fortune.