You have confused the verb "to get," with the verb "to go." "I knew Sam for about two years before he got married." got = simple past of "to get." (gone = past participle of the verb "to go.") In your final sentence you have tried to use the pluperfect tense. This tense is used for completed actions in the past where two or more actions are described and one is dependent on the other. For example: "Sam had planned to marry Rachel, but then he changed his mind and married Jane instead." In the sense that you mean, your final sentence should just have used the simple past tense: "I already knew Sam and Rachel before they got married."