Ultimately you have very little control over how members of a Group are displayed on the sitemap when using the Group element. I’m not certain @dim’s recommendatins will work or not as I’ve not done any tests or looked at the code to see how it determines the order but I’m sure you will find soon that you will want to do things like hide Items based by value, change its color based on value, change how it is displayed (e.g. display a Switch as a Text element) insert a Chart, etc. None of this is possible using the Group element in the sitemap.
Even though it is more work, I recommend using 
Text item=gFirstFloor {
Frame {
// list each Item individually with customizations
}
}