So if you want to use the normal hawk as a ranger pet, the most straightforward way of going about it is to create a ranger, select some pet, and then use the console to manually remove the pet you selected, and manually add the gunhawk. If you dont want to disable achievements, you can probably use the Unity Console mod.
Alternatively, you can try this test mod I just made. With it, you should be able to create a new ranger character and be able to select the bird pet right from the start. I am not an experienced modder, however, and cannot guarantee that this will work flawlessly. I did give it a quick try though, and it seems to work as it should.
To install it, download the file and change it from "test.txt" to "test.gamedatabundle", then go into PillarsOfEternityII_Data\override\, make a new folder in the override folder (name it whatever you want), and put "test.gamedatabundle" in that new folder.
test.txt