Just checked geck don't have an option to spawn object by explosion, but FO3edit has this option. So you'll have to use FO3edit for it and write the script in geck. To create a new projectile in FO3edit, just pick a vanilla projectile, rclick and choose copy as new record, follow the instruction to rename it and dump it in your file or a new file, do the same thing with explosion. Now you have custom projectile and explosion, In projectile property: click FLAG to edit it, put a tick next to explosion. Once you done edit the explosion box in projectile to point to your custom explosion. In explosion property: look at the bottom INAM placed impact object, edit it to point to your marker object. Hope it helps, you can also dl some other mods (satellite nuke strike...etc) and look into their file, thats how I learn about it. Hope it helps.