hi alpha,
yes, you want to keep anus dry within reason. so using the gauze (whatever size will work, 3x3, 4x4, etc.) or even if you want something softer (although it's not as thin), you can buy cotton laid out in a roll - you just have to cut squares (i never saw any pre-cut for you).
so after you take your hot/warm bath with or without epsom salts, then you dry your bum as much as you can and then you place the gauze in between the cheeks and amongst the anus (no need to press it into the anus)....this will help you also visualize as the days go by how the post-op discharge is getting smaller - also a great way to visually inspect any changes in size, color or smell of discharge.
as i mentioned before, i bought diapers to also catch any "incontinence" leakage/BM post-surgery but i never had any issues with that. so after day 3 or so, i stopped wearing the diapers. kept the gauzes.
when you go back to work, and if you walk like i do to work or in the middle of the humid summer, you may want to bring some extra gauzes with you to change in the middle of the day to keep the moisture down.
***
you keep wearing the gauze when you barely see any discharge or when you feel no longer necessary. i'd say at least a good 3 weeks or until your first post-op check up!