I take 'which' to refer to the whole preceding clause.
If it just was meant to refer to gxf3, it would typically be written another way, eg Black will open up the g-file with ..g5-g4, followed by ...gxf3 which attacks the base of White‘s **** chain.
You mean Black will open up the g-file with ..g5-g4 followed by ...gxf3, which attacks the base of White‘s **** chain. is equivalent to Black will open up the g-file with ..g5-g4 followed by ...gxf3; a plan which attacks the base of White‘s **** chain. ?