a) 逆 (ni4) - Japanese (gyaku) - reverse, contrary and opposite
b) 突 (tu) - Japanese (Tsu) - thrust, stab and suddenly (abruptly) move
c) き(ki) Japanese Hiragana ideogram - type, theory, tree, wood and indicator
With regard to the Hiragana 'き' (ki) ideogram - in Katakana it is expressed as 'キ' (ki) - an ideogram used to transliterate foreign terms into the Japanese language - particularly those of Southern Min origin. This concept seems to mean 'reverse thrust type' or 'reverse thrust technique'.
Goju Ryu Self-Defence
However, as there may be a play on words in operation here, it could be that 'gyaku tsuki' means that the power of this technique is gained through the agency of 'moving quickly' - and then 'suddenly stopping' stock-still - perhaps 'solid' and 'unmovable' like a 'tree'!