Annotate specific text in pdf using ghost script
You're going to have to be more specific. What do you mean by 'annotate specific text' ? Are you talking about;

Altering a FreeText annotation ?
Finding text in a content stream and altering it ?
Adding an annotation at a specific point on the page ?
Something else ?

Ghostscript is not intended as a general purpose PDF editing tool, and so it's quite limited in what you can do with it. You might be better advised to use something like MuPDF, but so much depends on exactly what you are trying to do, it's not really possible to make any specific recommendations.
I am trying to extract entities from the pdf and mark it back to pdf using pdf annotation. 
Is there any way I can annotate specific text using ghostscript?
Thanks