[General Questions and Answers] RMN9PRO5G_User:
I experienced the same - after update v12.0.3, Gapps stopped working. I then searched for solutions and was able to flash it back with ROM version before the update. What I did was:

1. Enable the Developer Options on your Redmi Note 9 Pro 5G.

Go to Settings>My Device>All Specs>Tap 5 or 6 times on the MIUI Version/ Build Number until you see a pop message that shows you “You are now a developer”.

2. Head to the Settings>Additional Settings>Developer Options>enable USB Debugging and OEM Unlock.

3. Go to Settings>Developer Options>Mi Unlock Status and then:

a.Agree to Permission

b.Click Add account and device

c.Turn off wi-fi and use mobile data

d.Your device will be "binded"

3. Using a computer download Mi Unlock tool. In my case I downloaded it from https://www.gizmochina.com/2020/02/11/download-xiaomi-mi-unlock-tool/:

a. The version I first downloaded is miflash_unlock-en-3.5.1128.45.

b. Install it. In my case, the USB driver for my device was also installed.

Another website to download the unlock tool is https://en.miui.com/unlock/download_en.html. This is the version miflash_unlock-en-5.5.224.24. In my case, it was the first version installation where my USB driver was installed. But the second version was the one I used to unlock.

c. Run the unlock tool.

d. Enter you Mi Account credentials. You may need to enter the code sent to the Phone number registered to your Mi Account.

4. Power off your device. Long press the volume down and power button until you see the Fastboot screen. Connect your Device to the PC and then:

a. Click Refresh. Unlock tool shall now show your device and then click unlock.

b. After a short while, you will be advised to unlock after few days, in my case it's 7 days. (It is important at this point that you already creatde a Backup and copy it to your computer. Copy separately the DCIM Folder).

5. After the waiting period:

a.Power off your device. Long press the volume down and power button until you see the Fastboot screen. Connect your Device to the PC.

b. Run the unlock tool.

d. Enter you Mi Account credentials.

e. Click Refresh. Unlock tool shall now show your device and then click unlock.

6. Download Mi Flash Tool. In my case, its MiFlash20210226. unzip and install it.(Note: download the Mi Flash Tool in advance so you have it ready. You may download it same time when you download the unlock tool).

7. Download the Rom Version before the update. I downloaded it from https://xiaomirom.com/en/rom/redmi-note-9-pro-5g-mi-10t-lite-gauguin-china-fastboot-recovery-rom/. In my case, It is a history Fastboot Rom since it is a rollback. It is gauguin_images_V12.0.11.0.QJSCNXM_20210219.0000.00_10.0_cn_01867c5953.tgz. Unzip it and place it in drive C of your computer. (Note: download the Rom Fast Boot in advance so you have it ready).

8. To prevent anti roll back error, I edited all the .bat files. I renamed them to .txt and deleted a portion that restricts roll back. In my case, I deleted the portion after the 2 lines that starts in "fastboot %* getvar..." The portion that I deleted starts with "::check anti_version" and followed by 4 lines of "if..". After delete, you will see that all the lines now start with "fastboot %* only. I then rename it back to .bat. You may want to check out this youtube video:

8. Run the Mi Flash tool and then:

a.Power off your device. Long press the volume down and power button until you see the Fastboot screen. Connect your Device to the PC.

b. Click Refresh. MiFlash tool shall now show your device.

c. Click Select button and find the folder of the ROM in your Drive C. VERY IMPORTANT: at the bottom of the flash tool window,you must select "Clean all" before you click flash button.

6. Wait patiently while flashing. After flashing (about more than 500+ seconds) your device will reboot. Note that the flash tool may say error (something about lock) but in my case, flashing was successful even if it said error. Your device will now reboot. MIUI may take a while to load. Wait patiently. You may need to register again using your MI credentials. It may be in chinese text but you will know because it is shown first after boot. After that, it will all be setting up like its a new phone (you can now choose English language).

IMPORTANT ADVISORY: It is important that you create a Backup and copy it to your computer. Unlocking already deletes the data of your phone. After flashing, you may copy it back to MIUI>Backup Folder>All Backup and use it to restore your contacts and other settings. Also always make sure your phone battery is 80% full if not 100% when unlocking or flashing. In my experience, China Rom is still best to use for Redmi Note 9 Pro 5G because it maximizes the use and features of your phone.Other ROM (India or Global may cause bugs and camera issues). Only caveat is, you can no longer update your MIUI. As the updated MIUI is the one that stops the Gapps from running in the first place. After all this, you will appreciate how unique, special and one of a kind phone is Redmi Note 9 Pro 5G.

UPDATE JUNE 1 2021:

Guys, just right now, after checking, my phone showed MIUI 12.5 as available update. Fingers crossed, I updated my Redmi Note 9 Pro 5G to MIUI 12.5 (v12.5.2.0), and like a blessing from heaven, all my doubts, fears and worries of not being able to use GMS are gone. After updating with v12.0.3 before, all my Gapps stopped working. But with this update, you just have to go to Settings, Accounts and Sync, Basic Google Services (this is new) and just enable it. And just like that, all that nightmare about Redmi Note 9 Pro 5G China rom not allowing google apps is gone. MIUI 12.5 is so smooth and awesome, so feature-rich. For me, sticking to China Rom (english language though) is still the best for China rom phones for you to be able to maximize and enjoy all its features. Flashing other roms may cause bugs and camera issues. Salvation has come! :D I have no idea about other models but I think K40 also has available update already.