I was just wondering how to get a gui to open when a block is powered. I know how to do the gui, but not checking when the block is powered. any help?

Wow cool idea! Its not that hard!

 

If you use an TileEntity than its very easy:

 

public int delay = 0;
public void updateEntity()
{
     if(worldObj.isBlockIndirectlyGettingPowered(xCoord, yCoord, zCoord))
     {
            EntityPlayer player = worldObj.getClosestVulnerablePlayer(xCoord, yCoord, zCoord, 3); //Gets the clostet player in a 3 blocks range;
            if(player != null)
            {
                  if(delay >= 100)
                  {
                        delay = 0;
                        player.openGui(modInstance, GuiID, xCoord, yCoord, zCoord);
                  }
                  else
                  {
                        delay++;
                  }
            }
            else
            {
                  delay = 0;
            }
     }
     else
     {
           delay = 0;
     }
}

The Delay is needed that the player can still close the gui.

Else the spam would be bad.

But thats all code. You search for a redstone signal.

Get the player (closest) check if its not null (safe function)

than you open the gui.

I hope it helps'