Yes, Thank God it's Friday would be the best translation for Δόξα τω Θεώ είναι Παρασκευή
It's a feminine noun meaning (roughly) "glory", here in the nominative case.
τω Θεώ is in the old dative case, which has died out in spoken Greek but lives on in a handful of phrases.
So this is literally something like "glory to God", with a jussive(?) "(may there) be" (έστω) understood, i.e. "glory be to God".