It is central and can be seen from both sides.
The gravid spot can be seen from both sides of a female guppy, but it is located directly in front of the anal fin on the bottom of their abdomen.
The gravid spot on a guppy is not always a 100% determination of pregnancy as the colours can vary. Quite often, when they are ready to give birth, the gravid spot becomes quite dark.
If your female guppy is being or has been kept with a male then she will be 'gravid'. The colour of the 'gravid spot' varies.
The gravid spot. It indicates that the guppy is pregnant, and the gravid spot will grow larger the farther the guppy is into pregnancy. (pregnancy usually lasts about one month)
There will be a dark spot in front of the fishes anus (cloaca) in a pregnant (gravid) female livebearer. This spot is called the "Gravid Spot".
The Gravid female will have the dark "gravid spot" in front of her vent. I can however assure you that if there has been a male in the same tank with a female guppy for any reasonable length of time she will be gravid.
When your female guppy is pregnant you can't tell by it's size. You have to look at it's gravid spot. Which is right behind their stomach and the tail. It's right around the butt of the guppy. It takes about a month for the babies to be born. Plus you can not stress out your the pregnant fish.
Many male guppies have 'black spots' but if by "black spot" you mean "gravid spot" then no, a male guppy can not be 'gravid' because gravid is the equivalent of "pregnant" and male guppies can not get pregnant.
That is quite common. Don't worry about it. A gravid spot does not have to be dark or black. Your guppy is perfectly normal.
Try explaining why it's there.. it means the fish is pregnant. The darker it gets the closer she is to giving birth.. The gravid spot on a female guppy is a dark area directly in front of the anal fin.
You have said she has a gravid spot (not gravel). That indicates that she is gravid (pregnant). She would not have that spot is she was not gravid. Be patient. Provided she is in suitable conditions she will have her youngsters eventually.
You will have to go by the size of their tummies. Once a guppy starts breeding she will always have a gravid spot.