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.
All that is needed is a male guppy in with her.
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.
She could well be ready and waiting to release her yougsters.
A pregnant female will have a black spot near their tail that continues to grow in size as she gets closer to giving birth.
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.
Depending how old it is, if it is an adult female and it is in with male guppies then it is very likely that it is pregnant. (:
it will have a red dot and black on its belly and will be fat
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.
female guppies can produce 2-3 genaration in a year
The female guppy will develop a dark spot on the side of her belly during pregnancy. As the pregnancy progresses, the spot will grow darker.
there is not much difference but it is actually not called a fancy guppy its called a fancy tailed guppies because they have beautiful tails but beware if you have a male and female guppy or fancy tail guppy in the same tank the female will get pregnant and after the babies are born you must separate them or most of the babies will be eaten but beware the babies you can hardly see.
The black spot will not go away as it is the eyes of the fry through the pale skin of the guppy. As the pregnancy progressess, the (black) gravid spot will get darker, and the fry will mature.