The GEN_VAL1 is a real time field, which implies that historical data is not available within Eikon data API. If you open the data item browser, it will show that this field does not have a "series checkbox" in the parameters.
If you are interested in historical data, other products like Refinitiv Tickhistory can provide this.
I checked and found that 140 is a value of FUT_BASIS in the RDP historical-pricing endpoint.
Typically, the get_timeseries method in the Eikon Data API can retrieve history data of Open, High, Low, Close fields.
As mentioned by my colleague, you can try other products, such as Refinitiv Tick History or Refinitiv Data Platform to get historical data of FUT_BASIS.