D&D Physics prevent this from happening
Pressure isn't really a thing in baseline D&D 3.5. The Water on the Plane of Water is part of the Plane's [Water] trait, and the Gate spell doesn't change Planar traits.
Four basic elements and two types of energy together make up everything. The elements are earth, air, fire, and water. The types of energy are positive and negative.
The Material Plane reflects a balancing of those elements and energies; all are found there. Each of the Inner Planes is dominated by one element or type of energy. Other planes may show off various aspects of these elemental traits. Many planes have no elemental or energy traits; these traits are noted in a plane’s description only when they are present.
The Material Plane seeks to balance all the elements (and, well, pretty much everything else, actually), so it's possible a Gate might let stuff from the outside in, there, in moderation. The Material Plane doesn't allow elemental excesses, though, normally, so the flow would have to stop at some point. A Gate to the Plane of Water might pool water around it on the Material Plane side, drip, or do nothing at all, at the GM's discretion, whichever best reflects the Plane's careful balancing of the elements.
The Plane of Water side, however, is strongly [Water] dominant, and certainly won't lose any water/become less water domininant, even if water appears to pass through to the other side.
In my games, I allow characters a Knowledge(Planes), Spellcraft, or Sense Motive check to identify the planes linked by a Gate spell and how they feel about eachother, just the planes linked, or just the relationship respectively. It's probably important to remember that Planes have alignments and stuff, even if they aren't fully sentient (which some of them are). Furthermore, "Except for rare linking points, each plane is effectively its own universe with its own natural laws."