Place 1/2 cup of grape juice in a small bowl. Sprinkle the gelatin on top and whisk it in. Allow it to sit for 10 minutes so the gelatin softens and absorbs some of the juice. Microwave the bowl for 20 seconds to liquefy the gelatin. If it's not fully liquid and clear after 20 seconds, microwave in 5-second increments just until melted. You don't want the gelatin mixture to get hot.
Combine the remaining grape juice and the ginger ale in a large measuring cup or pitcher with a spout. Whisk them together, then continue gently whisking as you pour the gelatin in. Pour the juice into 16 shot glasses, leaving a margin at the top since you'll be adding eyeballs later.
Refrigerate the shot glasses for about 30 minutes—check their texture every 10 minutes. You want the gelatin to start getting thick, like cake batter texture. It shouldn't be set and firm, but it should be thick enough to suspend an eyeball in the middle of the glass. Depending on the temperature of your ingredients and your refrigerator, this process can take anywhere from 30-50 minutes. Just check the glasses regularly and don't be afraid to check the texture with a spoon or with your finger.
When the gelatin is thick enough, press an eyeball into the middle of the shot glass. You can use a thin knife or spatula to push it down into the gelatin, or just use your finger. Try to position it against the front glass, so it shows through clearly. Add a second eyeball, if you'd like. Continue until all of the shot glasses have floating eyeballs.
Chill the glasses until the gelatin is completely firm, about 2 hours. These Jell-o shots can be made several days in advance. In my experience, the gummy eyeballs did gradually absorb moisture, and were at their best within about 3 days of assembly.