St. Gertrude has much to say on these souls and we can join her in praying for them. Years ago, I made the great offering of my merit to go to them, as I loved these souls even as a child. My merits would fill a small tea-cup, but the Crucified Saviour merited heaven for these just.
Today, on the Feast of St. Gertrude, pray for a specific soul and have Masses said for those who have died. I pray for specific people who have died, rather than a general prayer.
Perhaps you could choose someone who has no family to pray for them. Or, a person who seemed not to have died in the Faith. God is merciful.