A "Gantry billboard" means a billboard fixed to an overhead structure, usually spanning a road. It is a double-sided structure, therefore offers the advertiser the option of advertising to either side of the traffic.
Gantry as a medium of advertising is highly advantageous because it attracts the attention of your potential buyer that too within a span of few seconds. Gantries may also display useful directional information and hence they are difficult to miss! They deliver high impact and allow for an effective long term presence in the market due to their prominent positioning and large display size.