LOL...this is too funny!! There isnt a Target on 15 and Gratiot. There is one on 23 and Gratiot. At 15 and Gratiot there is an Old Country Buffet that I really like to eat at, but other than that I dont think that there are any other eating places at 15 and Gratiot. There is also a Target on Gratiot and I think between 14 and 13 mile....if I remember correctly...all I know is that it isnt on 15 and Gratiot.
Hope that helps.
Gratiot also has a lot of fast food restaurants around the 16 mile area. That would also be another good spot to put a Sonic.