The Land Gantry is a structure common to the UEF and the Cybran Nation. It constructs all land-based experimentals. It is unlocked along with the first relevant experimental researched.
The Air Gantry is a structure common to the UEF and the Cybran Nation. It constructs all air based experimentals. It is unlocked along with the first relevant experimental.
The Experimental Gantry is a structure unique to the Illuminate. It constructs all mobile experimental units and is much larger than it's UEF and Cybran land-air counterparts. It is unlocked along with the first researched Illuminate experimental.
Since each Experimental Gantry can construct all Experimental Illuminate units you have unlocked, your opponents won't know if you will be attacking from Land or Air until construction begins. With proper scouting, though, a canny opponent can learn exactly what is being built, as the unit is clearly visible during the entire construction process.
On maps like Geothermal Borehole where base space is at a premium, the Illuminate Experimental Gantry is superior because you otherwise simply may not have room for both an Air Gantry and Land Gantry.
All experimental units can be launched "Half-Baked", causing them to enter play at half health and with some miscellaneous problems as well (indicated by a small lightning bolt on their model) This problem can NOT be fixed, so its not recommended to use this ability for mass production
It is not recommended to send out half-baked models without fixing them, as the malfunction condition they suffer from reduces their effectiveness greatly, and is irrepairable.