A few critical pieces of kit for wet gravel events:
Castelli Gabba long sleeve. Adjust base layers for the temperature.
Fenders - at least a minimal rear fender to block your saddle region. Full fenders are amazing for keeping you clean and dry if you aren't riding in a group.
Gloves with a cloth patch for clearing the glasses.
Don't use knee/leg warmers. Three valid choices are winter cycling tights, running tights over the bibs, or embrocation.
Lake boots (if it's cold enough to justify them).
Tall wool socks.