Please I need an alternative drug that perfectly substitute Gentamicin injection?

No single substitute. Gentamicin is a powerful antibiotic that can have serious side effects. It is advisable to do a culture of the bacteria causing the infection and test sensitivities in the lab to see which antibiotics will be effective. Bacteria sensitive to Gentamicin will usually be sensitive to other antibiotics; then choose the safest alternative option. Avoid fluoroquinolones like Levofloxacin if possible.