It is a gallows. The Book of Esther confirms this at 7:10 where it says:
וַיִּתְלוּ אֶת הָמָן עַל הָעֵץ אֲשֶׁר הֵכִין לְמָרְדֳּכָי וַחֲמַת הַמֶּלֶךְ שָׁכָכָה:
And they hanged Haman on the gallows that he had prepared for Mordecai, and the king's anger abated. (JPS translation.)
The root of the word יִּתְלוּ is לתלות which means "to hang." Also, I'm pretty certain that if the text even implied "impaled" that the rabbinic discussion of the text in the Talmud tractate of Megillah would have been all over that, just as they discuss many interpretations of other terms never considered in Christian texts.
Jewish Midrashic texts make it clear that the rabbis understood only that Mordechai was hung from a gallows. For example, Midrashic literature discusses Zeresh's recommendation to hang Mordechai as the best choice of execution for Haman. Mindy Kaminker writes:
The sources describe Zeresh as a very wise woman who even knew the secrets of sorcery.7 According to the Midrash, Haman had 365 advisors, but Zeresh’s advice was the best he received. She found an original way to kill Mordechai, one that had never been tried, telling her husband: "You must remember that Mordechai is a Jew. If you try to kill him with a sword, know that Pharaoh attempted to decapitate Moses and failed. If you wish to stone him, remember how David slew Goliath with stones. If you try to drown him, remember how G‑d tore the sea before Israel. If you want to exile him to the desert, remember how Israel wandered in the desert for forty years and thrived. Joseph was released from jail and became the viceroy. Chananya, Mishael and Azarya went out from a fiery furnace, and Daniel left the lion’s den. Don’t try to blind him; remember how many people Samson killed whilst sightless. There is one remaining way for you: hang Mordechai on a tree" [citing Esther Rabbah 9, Yalkut Shimoni 6:10057].
According to the midrashic source, Agadat Eshter 5:14, Zeresh went with Haman to pick out a pole or tree from which to hang Mordechai. The pole they chose was a relic from Noah's Ark, according to the Yalkut Shimoni (Esther 6:1056).
Not only does Jewish tradition accept that Haman was hung by his neck, it is exremely unlikely that the authors of the Book of Esther, the Sages of the Great Assembly, which included Mordechai, Ezra, Daniel, Nehemiah and Zechariah (see Babyl. Talmud, Bava Basra 15a), would have used the word לִדְקוֹר (to impale), especially since that is the word used at Numbers 25:8 to describe what Phinehas (Pinchas) did with his spear to the copulating Baal Pe'or priestess and Zimri ben Salu (done in such a way as to preserve the evidence that the two were copulating at the time of death -- for explicit details see Bably. Talmud, Sanhedred 82b). Because the Sages of the Great Assembly were not ones to use a vague word when an explicit word would do, I can find no reason to believe that Haman was impaled.