Is a woman who was unaware of the obligation of ghusl other than for menstruation and intercourse, and who used to experience wet dreams without performing ghusl, contenting herself with wudu for prayer for years, considered sinful?

Prayer without purification is invalid, whether it is major purification (ghusl (definition)) or minor purification (wudu'). Major ritual impurity (janabah (definition)) can only be removed by ghusl, for the Prophet, peace and blessings be upon him, said: "Allah does not accept a prayer without purification." Therefore, the prayers you performed are not valid, and you must make them up. There is no sin upon you for your ignorance of the ruling.