Hi,
1 : gnda is an arbitrary net name. In spectre you HAVE TO connect it to gnd (which is the compulsory 0V reference for calculations).
You can use agnd it to have different current return path in your ASIC (ie one from low noise analog front end : agnd , one for analog gnd, one for digital dgnd..., thus ensuring good EMC). Of course, outside your ASIC, all gnd are tied together (or not, but that is another long story...).
2 : parasitic reduction will not reduce parasitics in your layout, but filters the netlist and remove small (below a threshold) serial resistor and capacitor, thus reducind the number of node in the netlist, thus decreasing simulation time.
Hope this helps !
Regards,
RG