「顔{かお}はやや仰向{あおむ}きがち に、天{てん}の栄光{えいこう}をながめやる 目{め}が、深{ふか}くやすらか にみひらかれて いた。」
You ask:
Verbs suffixed by がち and then followed by に may function as an adverb, but then how does it relate with 顔, which is is followed は?
「仰向き」 is a noun here, not a verb. 「がち」 can be preceded by either the 連用形 of a verb or a noun.
「顔はやや仰向きがちに」 adverbially modifies the following verb phrase 「天の栄光をながめやる」.
「天の栄光をながめやる」, of course, is a relative clause modifying 「目」.
「目」 is the subject of the passive-voice verb phrase「みひらかれていた」.
Thus, the sentence structure is:
「(description of head/face positioning + に), (qualifier + 目) + が + (qualifier + passive-voice verb phrase みひらかれていた)。」
My own TL (which is not the main part of this answer):
"With his/her head mostly facing upwards, his/her eyes that gazed out at the heavenly glory were widely opened."