Is there a special rule for using the word "Gild" and "Guild" as verb or noun?
Guild, an association of craftsmen formed for mutual protection of business, has no action form. There is no verb "guild".
Gild, used as a noun can be the same definition, and can be used interchangeably with guild.
However, it also has a verb form, which refers in general to making something shiny in some form (specific definitions are listed above). This can't be replaced with guild, since that word has no verb definition.