when Transpositionist targets empowered Gangplank, Naval, can the opponent play a unit with gangplank at 6 might or at 9 might

9, not 6. Empowered Gangplank, Naval's replacement turns the "return me to hand" into "+3 [S] this turn" instead, so he never leaves the board and sits at 9 Might once the first instruction resolves (same math as FAQ #11757: 6 → 9). The linked second instruction ("play a unit with less Might than that unit had") is checked as it resolves, and since Gangplank is still on the board, his current Might is what's read — 9. So you may play a unit with Might 8 or less; a 6-Might unit qualifies, a 9-Might unit does not.

Two things worth flagging. First, the ability only returns a friendly unit, so an opponent can never point Transpositionist at your Gangplank — they'd have to control him themselves. Second, the "had" wording normally exists to look back at a unit that has already left the board (Rule 359.3.e.13); here there's nothing to look back past, because the replacement means no zone change ever happens. That reasoning is my interpretation — there's no official FAQ on this specific interaction.