Steps to Install Android 2.3 Gingerbread on HTC Desire Z
1. To flash or install ROMs on your HTC Desire Z, you need a rooted device. Make sure you have root access on your Desire Z before you proceed further. Root it first if you don't, using hboot.
2. Search for ROM Manager on the Android Market and install the Clockwork Mod Recovery ROM Manager.
3. Also Download the Android 2.3 Cyanogen Mod 7 ROM for your HTC Desire Z. You can get the CM7 nightly build.
4. Copy the ROM to your phone's SD card root. Start ROM Manager on your phone and click on Flash Clockwork Mod Recovery to flash a recovery image. Make sure the version is 3.0+. If it isn't, download the Clockwork mod recovery 3.0 image from here, rename it to recovery.img and copy it to the root of your SD card.
5. Launch ROM Manager again and select the Install from SD Card option. Select the Android 2.3 Gingerbread CM7 ROM from the SD card and install it. Back up your existing ROM and wipe data and cache before you flash the Android 2.3 ROM.
6. Your Desire Z will reboot and start the flashing process. The ROM will now be installed on your HTC Desire Z. In a few minutes, your HTC Desire Z will be running Android 2.3 Gingerbread.
Note: Flashing ROMs can be a risky procedure, so continue at your own risk. If you have any doubts or queries regarding the process, check out the XDA thread and post there, or leave a comment here.
You can download Google Apps from here and flash it too.