δωρεὰν [2 verses](adv)  "Freely" is from dorean, which means "as a free gift," and "undeserved," from a root that means "gift" and "present." - "Freely" is from the Greek adverb that means "freely" and as an adjective, "as a free gift," and "underserved," from the word for "gift."

δῶρον [8 verses](noun sg neut nom) "Gift" is doron, which means "gift," "present," and specifically a "votive gift" or "offering" to a god. The simpler term without the sense of a votive offering is "dorea."  - The word translated as "gift" means "gift," or "offering" but it has the special meaning of an offering to the gods. These gifts were "burnt" offerings, offerings that were burnt upon the grill that was the altar.

δουλεύω (3 verses) (verb 1st sg pres ind act) Do I serve," is from douleuo, which means to "be a slave", "serve", "be subject", "make oneself a slave", "accommodate oneself," and "render a service." -- The Greek verb for "do I serve"  means "to be a slave." It is the verb form of the noun translated as "servant", which means "slave" or "bondsman". 

δοῦλος [56 verses](noun sg masc nom) "The servant" is doulos, which means a "slave," a "born bondsman," or "one made a slave." -- The noun translated as "servant" means "slave." It is translated as "servant" to update the Bible.

δόξῃ [26 verses](noun sg fem dat) "Glory" is doxa, which means "expectation," "notion," "opinion," "repute," and "popular repute." Translations as "glory" or "splendor" are found primarily in translating the Bible. The words "recognition," "honor." and "reputation" come closest to capturing the Greek word, but Jesus uses it only in the most positive sense so "prominence" may come closest. -- (CW) The Greek noun translated as "glory" means "expectation," "notion," "opinion," "repute," and "popular repute." Translations as "glory" or "splendor" are found primarily in translating the Bible. Though it can have both a positive ("shining reputation") and negative ("bad repute") in Greek, Jesus only uses it to describe the word's "magnificent" aspect. The verb form has the sense of "recognize," but "recognition" while positive in the sense of rewarding people simply means knowing them in the noun form. See this articlefor more. CW --Confusing Word -- The "glory" does not capture the word's more general meaning.