As mentioned in some comments, this will depend on the setup your employer has on the pc and in the network you are working on. You are in your rights to be concerned about your privacy and we all should always keep an eye in the correct use of personal data even if it's something simple as our search results and youtube video history.
If your gmail accounts are synchronized in the browser, all your synchronized settings will be in all of your devices. If you are just logged in, you will need to access account settings and view history from there. This means that whoever has 'physical' access to a device with your account can see your activity. This does not mean that your employer has access to it even if that info is on the work computer, unless he walks to the computer and uses it.
But there are malicious ways to gain more information or access to your accounts like a remote monitoring tool. If your work pc has a remote tool, then he can get access to the PC remotely and view your stuff since the accounts are already logged in.
Another way can be to have a keylogger installed in the background and everything you type will be recorded by the employer. They can get passwords and other information from you.
Another malicious way is that the network has some type of proxy (mitm) which all network traffic goes through. This way they can get your password, cookies or any other useful stuff. This way they can impersonate you on a different device and gain access to your account.
So if you want to be 100 percent sure you are not being monitored, check all your account settings, your connected devices, check for malicious software and backdoors in the pc... Then use a VPN when you are at work.