Suppose I have a list or map in controller, so how can I access them in GSP view using render method? def map = [name : "akshay"]
def name_map = [full_name : map] //map
def list = ["c", "c++", "java"] //list
//want to render above map and list to gsp and want to display it on page

render method accepts a map. In your controller provide the view name you want to render and a model map containing your data and then you can directly access the model map on your gsp.
def map = [name: "akshay"]
def name_map = [full_name: map] //map
def list = ["c", "c++", "java"] //list

render view: 'index', model: [nameMap: name_map, langList: list]

In your gsp, to iterate over list:
<g:each in="${langList}" status="index" var="lang">
    ${lang}
</g:each>

and to iterate over map of map:
<g:each in="${nameMap}" status="index" var="fullNameMap">
    ${fullNameMap.key}
    <g:each in="${fullNameMap.value}" status="subIndex" var="map">
${map.key}
${map.value}
    </g:each>
</g:each>

Also a word of advice, if you know the datatype of a variable in advance then use the datatype instead of def.