Is it necessary to mention the gender of the deceased (male or female) when praying over them?

There is no harm in informing about the gender of the deceased when praying over them, so that the congregants may offer the appropriate supplication for them. However, if the gender is not disclosed, the congregants intend to pray over the present جنازة (bier/corpse), and their prayer is sufficient, whether they supplicate using the masculine form, "اللهم اغفر له" (O Allah, forgive him), or the feminine form, "اللهم اغفر لها" (O Allah, forgive her).