It's probably possible to remove Pantheon/Granite and install GNOME on elementary OS, but it's also a bad idea. :) Installing GNOME would likely result in an unstable OS. In addition, apps that are designed for Pantheon/Granite may break in GNOME.
I'd recommend installing a version of Ubuntu that is configured to work with your desktop environment of choice. For LXDE, there's Lubuntu. For GNOME, there's Ubuntu GNOME. You can install another flavor of Ubuntu alongside elementary, run it on a virtual machine, or boot it from a live CD if you just want to do a test drive.
For way more details, see this question about installing KDE.