CentOS

My Centos 7 PC is crashing regularly and unexpectedly and giving following message as in the attached image

I don't see any errors in that screen, those appear like boot messages.

Maybe X crashed and dropped you to the shell? is the system usable at this point or is it completely frozen?

Yes, it is completely frozen.
It happens while I am using my gnome desktop.

You need to look at the journal logs, something is crashing, maybe the video card driver/module.

For example, the following command will show you all kernel logs from the PREVIOUS boot (the one that crashed):
or to see all logs and not just kernel, use:
maybe there is something there to give you a clue what is crashing.