What it does
"Grav-cannon" isn't a standard Magic keyword or ability—it doesn't appear on any official card or in the comprehensive rules. It may be a misremembered name, a card from an unofficial/custom set, or confusion with similar-sounding terms.
If you're thinking of something specific, a few possibilities: you might mean a particular creature or artifact with "cannon" in its name (like the various "cannon" cards that deal damage), or a custom/playtest card from a non-official source. There's also no mechanic by this name in any released Commander product.
To give you an accurate rules explanation, double-check the exact card name and set—search Gatherer or Scryfall. If it's a homebrew or proxy card from your playgroup, the answer depends entirely on the text printed on it, so share that wording and I'll happily break it down.
Iconic cards with Grav-cannon
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