What buttock muscle for intramuscular injection?

Gluteal means in the buttocks.

There are three muscles that more or less make up the part of the body that we call the buttocks; gluteus maximus, gluteus medius and gluteus minimus, hence gluteal.

IM means Intra Muscular.

So gluteal IM injection is given in the muscles of the buttocks.

The reason why an injection is given in the buttocks usually is that whatever the drug it has to slowly diffund into the bloodstream as opposed to IV (intraveneously) that is administered directly into the bloodstream. e.g. some psychoactive drugs needs to be administered this way to have a long lasting effect, so it is not necesarry to take the medicin quite as often.