If you format /system GAPPS will be wiped as it resides in.... /system. In this case you must reload GAPPS.
If you format /data then all the data (login information, etc) is gone.
If you're coming from anywhere beyond where I separated GAPPS from the build then you should not need to format /system or /data - update "in-place" should be ok. I noted this in the top posts if you read them.
However, no promises on that as I have no idea what you did with your phone. This is especially true if you have replaced ANYTHING in the /system area or run anything that tampers with the /system configuration, including things like the "V6 supercharger" and similar.
In other words if you're doing other than loading ordinary applications then what you did might be incompatible with the ROM in some way, and if you try to load over the top of that all bets are off as I have no way to know what you did and how it might interact with the base load.
I load over the top on my phone all the time - when I'm actively developing I might do it a dozen times in a day. If I KNOW there's an incompatibility I'll note it in the NOTES section in the first few posts, but in the general sense if you're following the upgrade path and are not tampering with the /system area yourself it's fine.
The thing is that most of the people here are tampering with the base configuration in some form or fashion for various reasons (e.g. minimum CPU speed, etc) and as such I have no clue what you're doing or what might be causing it. This ROM is what I run on my phone and while I'm usually a code build ahead of what's posted here because I'm working on it the version posted here is what I had on the phone a few days previous and was happy enough to use, or I wouldn't post it.
Thanks, that explains it