Yes it should go straight on.
You may need to fix the side brackets if they have snapped on your gsi bumper
Also pending on what phase your astra is you may have to change your headlights so that your grill fits properly.
If you have a v-grill now you more than likely have a phase 2.
ph1 and ph2 GSI bumpers are the same so that dont matter, the arch liners will not fit at the bottom as the GSI bumper flares out more. i drove around with out the proper GSI liners for a few months before i found some, so it isn't a must have to fit the bumper i just thought ill mention it.