I am now in Spain and have multiple Schengen visa I am interested to go to Gibraltar. Is it possible or need separate visa? Need to go from Marbella to Gibraltar.
From the official government page in Gibraltar, under section 2, subsection f:
The following persons do not need a visa for Gibraltar
(f) those nationals or citizens of Morocco, the People's Republic of China, Mongolia, India or Russia who are holders of multiple entry Schengen visas with a minimum remaining validity of 7 days on the date of departure. Such persons are only allowed to enter Gibraltar for a maximum period of 21 days.
So you do not need an additional visa, however, you need at least a week left on your Schengen visa, AND you can only stay in Gibraltar for three weeks maximum.
This is answered here:
You can find that page by following the link from the Wikpedia page that summarizes visa requirements for holders of Indian passports:
You will find that with a Schengen multiple entry visa, you do not need an additional visa if you meet certain restrictions (regarding the length of the stay in Gibraltar and the validity of your Schengen visa). In particular, you need to leave 7 days before your Schengen visa expires and in any case your stay is limited to 21 days.
As others have explained, your Schengen visa most likely allows you to enter Gibraltar. You also have to mind the fact that entering Gibraltar counts as an exit from the Schengen area. Since your visa is multiple-entry, that's not a problem for you but that's something other readers need to keep in mind.
Spanish border guards are known to be lax regarding the exit check at this border, partly for political reasons. If you don't meet one, you might want to seek one yourself to get a proper exit stamp. That way you won't waste any of your authorised stay in the Schengen area.