Squirting is a projection of fluid, like a squirt gun. Gushing is a flow of fluid, like you're pouring something out of a bottle.
They can also have different anatomical sources as well. Science doesn't have a solid answer on female ejaculation, but it's generally argued to either come from the urethra or the Skene's gland and is released in a narrow stream. Gushing can also come from these sources in the form of a flow of fluid that just runs down instead of projecting, essentially a weak squirt, but a gush can also come from the vagina itself as a flow of lubrication.
The fluid in a squirt/ejaculation is argued to be either urine (held by dissenting views) or some secretion from the various glands in the region, but a gush from the vagina would be vaginal lubrication.
In general, gushing is a broader term than squirting.
Studies on frequency of ejaculation (squirting) vary wildly, but generally suggest that about 10% of women can squirt on a regular basis, while about half of women do so on an irregular basis.
I'm sure studies on abundant vaginal fluid have also been done, though no numbers immediately spring to mind. My suspicion is that it's more common than ejaculation though, as I think most women wouldn't confuse it with an urge to urinate and hold back as a result (a commonly reported behavior from women in the subject of ejaculation).
Squirting has some distance to it, like peeing. Gushing produces a lot of ejaculate as well, but it doesn't come out at anywhere near as high a velocity, so it just kind of drips down the girls thighs.
Gushing would be more common that squirting although both are from an orgasm. The gush is more of our sex liquid seeping out of our p****** during arousal where squirting would be more like a forceful flow like peeing (which it is sometimes)