A gap filler medal is normally an officially issued medal which has a serious defect or damage or something is wrong with it to a point of making it of very little collector interest in its' own right. The term "gap filler" implies that an order, decoration, or medal was missing from a group and to "rebuild" the group, a medal substitute of some sort was needed to fill the gap in a row of medals. 

Don't expect much in quality or investment value from a gap filler - usually the defects are so severe that no one can be expected to list every nick or scratch. A detailed photo (minimum resolution of 500dpi) of at least both sides of the medal should be a standard request to observe the minute details that some collectors demand. By definition, there is no such thing as a "rare" gap filler but there might be a gap filler of a rare medal. Reliable sellers can provide further information and guarantees (avoid the ones that plead ignorance or it was found in their grandfather's attic or state that all sales are final!). 

The collector needs to be aware that some sellers may cloak the term "gap filler" with a phrase or word in the description which notes that their medal is not authentic - it may be a copy, repro, forgery, restrike, unknown, possibly, or whatever...  A fairly recent tendency in eBay (and the other auction sites as well) is to pass off a copy, reproduction, or forgery as "makes a good gap filler".  There is technically a difference between a gap filler medal and a copy medal that can substitute as a gap filler.  The underlying point is that this definition must be clarified when you hear it since it's one of those which appears to be changing with time. 

British Copy/Fakes/Forgery Medals

Medal specialist usually have a good working knowledge of gap filler medals within their area of expertise. The novice or intermediate collector should exercise extreme caution when this term is used (particularly on eBay, unless the seller is a noted expert). NOTE: The term "gap filler" is not unique to medal collecting or other areas of militaria so you might see it used in other ways - be wary of the change in meaning in each field of collecting.  Please visit my blog (http://blogs.ebay.com/tsa-li) and/or guides for further information.  Thank you for your visit!