She could well be ready and waiting to release her yougsters.
All that is needed is a male guppy in with her.
It might want to mate with it.
Guppy
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.
yes there are. that is how they get mates.
A female guppy has a darker brown line that is visible through its body. The male has a very light brown line.
That does not happen with guppies. They stay the same sex for life.
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.
Your male guppy is female and with child.
gold fish
If the fish has a "GONOPODIUM" it is definitely a male.
About 4 weeks.