G5M trans in a ZX2.
I am VERY familiar with those cars - a true expert.
ATF is recommended. But gear oil is also recommended in older shop manuals. Don't use ATF since there are so many better alternatives.
Use a dedicated modern gear oil. Dino Pennzoil Syncromesh works great. So does Red Line MTL or MTL/MT 90 mixed 50/50. I have Amsoil MTF [Synchromesh synthetic] in mine now. It is very nice.
Great shifting, and much better wear protection than ATF.
Get a 6 point 23mm socket. The drain and fill holes are the same size.
Use the forward facing fill hole to fill, NOT the VSS hole use a funnel and an extension from under the hood for filling [level ground].
3 to 3.5 qts will be needed, depending.