Bug: On my system, gnome-shell clearly "leaks" memory at a rate of ~2 MB/min...
Supporting data: I ran the following script upon logging in to Gnome, running from another tty (ALT+CTRL+F1) then came back to my graphical X running on tty8 and just let the mouse sit there idle. I stopped collecting data after 45 min as you can see in the graphic.
Here is the script I wrote to simply makes two columns in a logfile, one for gnome-shell's mem RSS and the second for a simple time stamp.
Code:
#!/bin/bash
for i in {1..6000}; do
number=$(ps -A --sort -rss -o comm,rss | head -n 3 | grep gnome-shell | awk '{ print $2 }')
correct=$(echo "scale=2;$number/1024" | bc)
echo "$correct $i" >> /dev/shm/mem_usage.txt
sleep 60s
done