PLEASE READ: Editing FastFlags could result in client issues (Not recommended by Roblox Staff to change them), do this at your own risk, although if it causes issues you can disable the changes.
Dislike the new Roblox CoreGui Update or just looking to disable it over all? Well, I’ve made a few simple steps to disable it for yourself.
Before we start, you will need Bloxstrap which is developed by Pizzaboxer. You can read his post below to figure out how to install it.
Once you’ve installed Bloxstrap, let’s open the menu. Navigate to FastFlags by using the sidebar.
You will see all these options, it may be confusing, but we’re only here to change a few things. Select the FastFlag Editor in the top left corner. Another menu will pop up, which is the editor itself. Next, click the Import JSON button.
Here, you will input the JSON file below in the code block. Please note that editing FastFlags is completely not recommended by Roblox Staff and you are doing this at your own risk.
Input the following:
{
"FFlagChromeBetaFeature": false,
"FFlagEnableInGameMenuChromeABTest": false,
"FFlagEnableInGameMenuChromeSignalAPI": false,
"FFlagPlayerListChromePushdown": false,
"FStringEnableChromePinnedChatForcedUserIds": "\"1;\"",
"FStringGameMenuChromeForcedUserIds": "\"1;\""
}
Click OK and it will load all the edited flags, to save these, click the Save button in the bottom right corner.
Woohoo! 
You’ve successfully disabled the new Roblox Chrome Update, please note that they could change this at any time and could by forced enabled regardless of FastFlags basically making this temporary. Enjoy it while you can. 