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.
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.
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.
If your female guppy has been in a tank/container with a male since it reached maturity it will be gravid (pregnant). Give it time. The fry will come eventually
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".
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.
The female gets a deeper tummy and there is a dark (gravid) spot by her 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.
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.
Under normal circumstances every female guppy you come across will be gravid (pregnant). My advice is don't worry about it she will be pregnant.
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.
The female guppy will be very large and will have a gravid spot near her anus. But when she is ready to give birth she will be very still and huddling near the heater in your tank.
With a guppy you need to look at the gravid spot. This is a dark spot in the fish's belly. A large gravid spot indicates the fish is likely to be pregnant. You can tell how pregnant the fish is by how large the spot is. Sorry I have no experience with other species.