You should be able to change units for the second semester up until the census date for it which would end up being until the end of the second semester. In terms of units, nothing is really concrete until the census date of the semester of completion. You just go to WES and change your enrolment and making sure you discontinue the one you want to get rid of, and adding the one you want and clicking submit! It'll show up amended on your my.monash a few days later
In re-enrollment for my 3rd year, I only put down 3 units for semester two, coz I plan on figuring out what units I want to do later on, probably after Sem 1 ones.
You'll need to be more specific with your last query about the gateway units though, coz its all this gateway stuff is new, and some of us oldies may not understand unless you be specific. What course are you doing, and what units are you looking at?