Although it seems like a good idea and even if the person was not diabetic it probably would not do any serious harm, it may also be illegal. Glucagon is a prescribed personal medication and if something happens to that person and you had (with good intentions) injected them with something and are not someone with any kind of medical training that within your scope of practice allows you to administer medication then you could be sued or in some very serious trouble. if you are a medical professional that allows you to administer meds within your scope of practice then administering glucagon might be covered within the "Good Samaritan Law" which is in every state, but the details tend to vary. But the problem still lays within that the glucagon you are administering is not prescribed to the person you are administering it to, regardless of your intentions.
The best thing to do would be to dial 911 immediately, let the operator and paramedics know that the person is a diabetic and allow them to do what is needed. the most you could do while waiting for medics is to do what you can to revive the person and try to get them some kind of oral glucose, sugar, juice, etc... at least from a legal perspective.