Gabardine is a little bit different than Ventile, so here I go with another one of my long, drawn out textiles post:
Gabardine is a twill weave, woven very tightly. It is proofed twice, once in the raw yarn and again as a finished product. Due to the nature of the weave, it is extremely water repellent as well as totally windproof. The nature of a close twill weave means that water is held by the fabric at extreme tension, preventing it soaking in too much. As well, a twill weave is very hard wearing, much harder to puncture than Ventile. This is why Levi's and other jeans are made with a twill weave. However, even the toughest fabrics rip or tear eventually and once a twill weave reaches that point it'll unravel fairly quickly.
Ventile is an oxford weave, also woven very close. Sometimes it's proofed, sometimes not. The biggest difference between Ventile and Gabardine is in the weave itself. An oxford weave is simply a basket weave of sorts, and when wet the weave allows the fibres to swell in a very uniform pattern something no gabardine will ever do. (That's the secret behind Ventile) About the only drawback of an oxford weave that I can think of is in terms of durability, it holes and rips much easier than a twill weave.
Cheers,
Adam