I would start from context.Months
to collect the required data for your grid:
var viewModels = context.Months
.Where(m => m.Month_Name == selectedMonthName &&
m.Vehical.Dept_Id == selectedDepartmentId)
.Select(m => new
{
Vehc_Number = m.Vehical.Vehc_Number,
FB_Id = m.Fule_Bill.FB_Id,
LB_GTAmount = m.Fule_Bill.LB_GTAmount,
LB_GTQuantity = m.Fule_Bill.LB_GTQuantity
})
.ToList();
viewModels
will be a list of anonymous objects with those four properties that you want to show in the grid. It should be possible to bind this list to the grid view.