over 句は対応してないみたいなので そこの select 列だけ knex.raw で書く
あとは普通に knex で作れる

const knex = require("knex")({ client: "pg" })

const query = knex.select("id", "group_id")
	.from(
		knex.select("id", "group_id", knex.raw("row_number() OVER (PARTITION BY group_id ORDER BY id DESC) as n"))
			.from("table1")
			.as("sub")
	)
	.where("n", 1)
	.toQuery()

console.log(query)
// select "id", "group_id" from (select "id", "group_id", row_number() OVER (PARTITION BY group_id ORDER BY id DESC) as n from "table1") as "sub" where "n" = 1