If "gush" means to speak enthusiastically and excessively:
* Be reserved
* Be concise
* Be restrained
* Be succinct
* Be quiet
* Be muted
If "gush" means to flow out rapidly and in large quantities:
* Dribble
* Trickle
* Drip
* Ooze
* Seep
If "gush" means to be effusive and sentimental:
* Be stoic
* Be aloof
* Be indifferent
* Be pragmatic
* Be detached
Example:
* Gush: "Oh my gosh, that dress is absolutely stunning! It's perfect for you!"
* Opposite: "That's a nice dress."
Ultimately, the best opposite for "gush" will depend on the specific situation and meaning intended.