וַיֵּצְאוּ֮ כׇּל־בְּנֵ֣י יִשְׂרָאֵל֒ וַתִּקָּהֵ֨ל הָעֵדָ֜ה כְּאִ֣ישׁ אֶחָ֗ד לְמִדָּן֙ וְעַד־בְּאֵ֣ר שֶׁ֔בַע וְאֶ֖רֶץ הַגִּלְעָ֑ד אֶל־יְהֹוָ֖ה הַמִּצְפָּֽה׃
Thereupon all the Israelites—from Dan to Beer-sheba and [from] the land of Gilead—marched forth, and the community assembled as one, before GOD at Mizpah.
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the Hebrew term containing אִישׁ.)
Prototypically, אִישׁ is used in sketching a situation schematically. The expression כְּאִישׁ אֶחָד, which appears nine times, describes the manner of action in an event, likening it to what would be expected if the situation had only one participant. Here, the point is that everyone who was roused to act left their homes and assembled in the same place. (Gender is not at issue.)
As for rendering into English, NJPS ‘to a man’ is an idiom that means “without exception; including every person” (Oxford English Dictionary, s.v. man, 17j). Although that rendering resembles the Hebrew expression in form, it misrepresents the meaning.
In contemporary English, man is not needed or used to convey the idea that everyone assembled in the same place. (The fact that women are not in view is self-evident from the military context, and thus can go without saying.)