Is it possible to use HOMER for modeling a Biomass gasification based electricity generation system?

You certainly can model biomass-fueled generators in HOMER.  There are two ways to do so:

1. If the supply of biomass feedstock is abundant throughout the year, the easiest thing to do is to simply define your own fuel (using the "New" button on the Fuel page of the Generator Inputs window) to represent the biomass fuel.  For example, you could define a fuel called "Producer Gas" with a carbon content near zero and the appropriate values for the lower heating value and density.  That fuel will then appear in the drop-down box of fuels, and any generator or boiler you model can consume that fuel.  (Be careful to specify the generator's fuel curve inputs properly whenever you switch fuels.  The Fuel Curve Calculator window helps with this.)  A button labeled Producer Gas will appear in the Resources section of the schematic.  Use that button to specify the cost of the producer gas.

 2. If the supply of biomass is constrained for all or part of the year, choose "Biogas" from the list of fuels for a particular generator, and when you hit "OK" the biomass resource button will appear in the schematic.  In the Biomass Resource Inputs window, you can specify the availability of the feedstock on a monthly or hourly basis, as well as the feedstock price and the properties of the biogas.