Depends on your front bumper support. I went through an issue with my 95. The fronts support was flat across and only works with the "L" bumper.
If you want to use the GT or the Outback bumper, then you need to make sure you have the crosspiece built with a lower section at the grill. Since your's is a 98 GT, you should be fine, but I have no idea why your bumper is missing, what "repairs" were done.
Best bet is to find a used GT or Outback bumper from a salvage yard, take a weekend and paint it using color match paint. Since we're so close to winter, let it dry and cure a few days before installation. :-) Don't forget to seal and wax the paint too!
Edit: Added link, there is a similar post from someone who just replaced their bumper on a GT: