Don't try to set the GUI display from within a block, if that's what you are trying to do. If you have an entry called "I_Gain", then there should already be a set_I_GAIN() function in the top block.
How to call set_I_Gain() depends on what you are trying to do. If you want to set it over the network or from another process, follow Paul's recommendation and use XMLRPC.
But, from the image attached, it looks like you just want to set the gain via a chooser and display it in the entry? In that case, just put "Change_I_Gain" (the name of the chooser) in the value field for I_Gain and it should be set whenever you choose a new value.