A gaarder is a collector, typically of taxes or tariffs.
You may come across the term doing research in the province of Holland between 1695 and 1806, when a special tax (“impost”) was levied on marriages and burials. The duties that had to be paid depended on the economic circumstances of the bridal couple or the deceased.
The records of those taxes are known as the gaarderboeken [collectors’ books]. These records can be used as substitutes for marriage and burial records. The actual marriage or burial dates can be a few days earlier or later than the date in the tax records. In my experience, most marriage taxes were paid between the date of the banns and the marrige, and mosts burial taxes were paid between the time of death and burial.
The research guide of the National Archives has a list of the gaarderboeken of the province of South Holland, that links to the finding aid with the imaged original records. Click the entry “Lijst met inventarissen van de Doop-, Trouw-, Begraaf- en Gaarderboeken van Zuid-Holland” and then select the record group number for the place of your choice in the “Gaarders” column.