Wrap your <p:outputLabel>
<p:outputLabel id="someText" value="Some text" rendered="#{controladorGestionGrados.value eq 'C'}" />
with a holder <p:outputPanel>
Like this :
<p:outputPanel id="someTextPanel">
<p:outputLabel id="someTextLabel" value="Some text" rendered="#{controladorGestionGrados.value eq 'C'}" />
</p:outputPanel>
And update the holder component (<p:outputPanel>
) with
<p:ajax update="someTextPanel" />
So entire code should be something like this :
<h:form id="form" >
<h:panelGrid >
<p:inputText placeholder="Name" value="#{controladorGestionGrados.otherValue}" />
<p:selectOneMenu value="#{controladorGestionGrados.value}" >
<f:selectItem itemValue="A" itemLabel="A" />
<f:selectItem itemValue="B" itemLabel="B" />
<f:selectItem itemValue="C" itemLabel="C" />
<p:ajax update="someTextPanel" />
</p:selectOneMenu>
<p:outputPanel id="someTextPanel">
<p:outputLabel id="someTextLabel" value="Some text" rendered="#{controladorGestionGrados.value eq 'C'}" />
</p:outputPanel>
</h:panelGrid>
</h:form>