My attempts to find out the origin of the word garmonbozia, and whatever ideas informed its use in Twin Peaks, surprisingly turn up nothing, just like in 2011 when watching the series for the first time, so I’ve decided to create my own. (All bold is mine)
Loosely speaking, “garmonbozia” is a negative spiritual energy of pain and suffering, or perhaps created from pain and sorrow. The bad spirits who inhabit the Black Lodge, such as BOB, intentionally manipulate people in Twin Peaks into negative situations in which they will experience emotional pain and sorrow, in order to generate garmonbozia.
The denizens of the Black Lodge are evil personified; they consume garmonbozia in order to instantiate themselves into corporeal form (or because of this).
One of the most plausible explanations is that it is derived from “ambrosia”, not the fruity dessert, but the “food of the gods” in Greek and Roman mythology. This is merely speculation, but fits well with what is seen in FWWM.
Shortly after seeing the series in 2011, I was watching a travelogue show of some kind where the destination was the various Baltic countries. Consider this local treat that was mentioned, with regard to the -bozia root:
Boza, also bosa (from Turkish: boza ), is a popular fermented beverage in Kazakhstan, Turkey, Kyrgyzstan, Albania, Bulgaria, Macedonia, Montenegro, Bosnia and Herzegovina, Azerbaijan and other parts of the Caucasus, Uzbekistan and parts of Romania, Serbia. It is a malt drink made from maize (corn) and wheat in Albania, fermented wheat in Turkey, and wheat or millet in Bulgaria and Romania. In Egypt where it is known as “būẓa” (بوظة) it is usually made from barley.  It has a thick consistency, a low alcohol content (around 1%), and a slightly acidic sweet flavor.
The etymon boza is also known from the Bulgar drink buzá, ‘a grey kvass-like drink’, borrowed from Turkish and perhaps the source of English booze, ‘an alcoholic beverage’ via Romani (cf. also Chagatai, Ottoman Turkic, etc.; boza, ‘drink made of camel’s milk’ and Chuvash pora, its r-Turkic counterpart, which may ultimately be the source of the Germanic beer-word).
Garmon- is probably the word hormone (гармон in Russian is what came to mind), but the root is greek:
1900-05; < Greek hormôn [ὁρμῶν] (present participle of hormân to set in motion, excite, stimulate), equivalent to horm(ḗ) horme + -ōn present participle suffix, with ending assimilated to -one
If gods consume ambrosia, then demons would consume ambrosia that was in some way perverted, so surely garmonbozia is a corruption of ambrosia, and given its purpose I think it a plausible kind of portmanteau, meaning “hormone-booze”. Maybe this describes David Lynch’s aesthetic adrenal overdrive too?