a!gridImageColumn is giving issue

If you have access to use the a!forEach function try that instead. I find forEach much easier to use for looping. Something along the lines of a!forEach(items: local!testsubset.data, expression : a!documentImage( /* code to determine the document to display*/ )). You shouldn't need to use the merge commands so it should simplify your code - I'd also suggest using proper documents to start with as I'm not sure the values you're passing in are actual document icons ?    

These are icons only. But by just changing if condition its not working. I tried to simply put "true" also in if condition not working.

Saying it should be list in merge function. I had put {} then its started saying document object is null.

I am not understanding why that is tightly bounded with this condition. I can't change this code pattern as of now.

Can you post the full code snippet as I've just tried to create a dummy interface in my sandpit and I'm getting an error message around 

Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function a!iconIndicator [line 16]: Invalid value for “icon”. Received: Hii. Supported values are:

I've now managed to recreate the issue locally - will see if I can help with the code.

I've experimented and have manged to create this logic - which works. See if you can adapt it to your use case. 

= load(
  local!testsubsetFinal: todatasubset(
    {
      {
        id: 1
      },
      {
        id: 3
      }
    }
  ),
  local!testsubset: todatasubset(
    {
      {
        id: 1,
        strDate: date(
          2019,
          05,
          30
        )
      },
      {
        id: 2,
        strDate: date(
          2019,
          06,
          10
        )
      }
    }
  ),
  a!gridField(
    label: "",
    columns: {
      a!gridImageColumn(
        label: "",
        size: "ICON",
        data: a!forEach(
          items: local!testsubset.data,
          expression: a!documentImage(
            document: a!iconIndicator(
              if(todate(fv!item.strDate) > today(), "FACE_HAPPY", "FACE_SAD")
            ),
           caption: if(count(local!testsubsetFinal.data)>0, 
            if(fv!item.id=local!testsubsetFinal.data.id,"Hi to all ","Hello to all "),"Date Implemented " & todate(fv!item.strDate)))
            
          )
        )
        
        
        
      
      
    },
    totalCount: length(local!testsubset.data),
    value: a!pagingInfo(
      startIndex: 1,
      batchSize: 10
    )
  )
)

I've experimented and have manged to create this logic - which works. See if you can adapt it to your use case. 

= load(
  local!testsubsetFinal: todatasubset(
    {
      {
        id: 1
      },
      {
        id: 3
      }
    }
  ),
  local!testsubset: todatasubset(
    {
      {
        id: 1,
        strDate: date(
          2019,
          05,
          30
        )
      },
      {
        id: 2,
        strDate: date(
          2019,
          06,
          10
        )
      }
    }
  ),
  a!gridField(
    label: "",
    columns: {
      a!gridImageColumn(
        label: "",
        size: "ICON",
        data: a!forEach(
          items: local!testsubset.data,
          expression: a!documentImage(
            document: a!iconIndicator(
              if(todate(fv!item.strDate) > today(), "FACE_HAPPY", "FACE_SAD")
            ),
           caption: if(count(local!testsubsetFinal.data)>0, 
            if(fv!item.id=local!testsubsetFinal.data.id,"Hi to all ","Hello to all "),"Date Implemented " & todate(fv!item.strDate)))
            
          )
        )
        
        
        
      
      
    },
    totalCount: length(local!testsubset.data),
    value: a!pagingInfo(
      startIndex: 1,
      batchSize: 10
    )
  )
)

Thanks for your response . I will try that to accomadate in my code.

You may need to iterate through the susbsetFinal.data.id using another for each but hopefully this will give you some idea of how it could be achieved.