These sort of cranes could be divided to several categories. The most usage of these sort of cranes is in outdoor conditions where there couldn't use fix columns and crane beams, so for achieving the goal, rails would be mounted on top of base plates which are fitted over an appropriate foundation.
In addition, in some cases gantry cranes could be a good choice for indoor conditions where the structure of saloon is not strength enough to bear the load of overhead crane, or applying columns & crane beams would be costly. 
If there wouldn't exist any restrictions for set up a gantry crane, this sort of cranes would be beneficial.

Gantry Crane Types:

Single Girder Gantry Crane
Double Girder Gantry Crane

Same as overhead cranes, single girder gantry cranes are usually common for capacities below 10 ton and spans shorter than 20 meter and double girder gantry cranes are common for capacities over 5 ton and spans longer than 12 meter.
 

Semi-Gantry Crane
Another type of gantry cranes is known as Semi-Gantry Crane which could be available both in single girder and double girder type and the main specification of them is that they could be installed near to a wall of saloon and at one side they have legs and their end carriage run over ground and at the opposite side their end carriage run over crane girders. This sort of designation could enhance to minimum space usage.

Notes to be considered for Gantry Cranes:

Design process, manufacturing and setup of gantry cranes are involved with so many crucial issues which ignoring any of them could lead to malfunctioning of final product. Some of those issues are listed below:

Distance between girders should be specified preciously
End carriage length should be sized preciously
Correct angle between columns should be specified
Checking columns against maximum stress and also probable buckling
Checking all structure parts against applied stress
Controlling all structure parts against maximum deflection
Best choice and design for connections
Applying in-service and out-service wind effect on the power of electro-motors
Controlling crane collapse against wind effects
Controlling crane collapse against earth quake effects
Applying sun-roof and rain cove for all electrical parts
Applying an access for PM
These sort of cranes require precious manufacturing and setup processes
Applying tolerances which are requested by relevant standards in manufacturing
Applying tolerances which are requested by relevant standards in setup

Specifications of Gantry Cranes

Quiet useful in outdoor conditions
Available both single girder and double girder
Shortening loading and uploading time
Available with single leg in single girder designation
Capability of using console for having access to the out of span area
Capability of using maintenance platform in double girder types
Capability of using Control Cabin in double girder types
Capability of using Control platform for operator
Various designations for any individual purposes