For me, the best way to get a pair of any cichlld, (Gymnogeophagus included) is to get a shoal of 6 or 8 juvies, raise them up together, and allow them to choose their own partners.
Just getting a male and female is no guarantee that they are compatible enough to mate and spawn.
I must admit I don't really think of Gymnogeophagus as predators, them are omnivores, but I suppose that's just semantics.
Any Uruguayan, or "southern" South American cichlid is a good candidate for an outside pond.
Australoheros sp. Ceibal (below) from Uruguay spawned in my ponds many times.
Below, Australoheros raising a brood in a 1500 gal pond, you can see the spots the fry have eaten in the substrate of algae and detritus
Below Gymnogeophagus quilero in the pond.
When I'd see a pair form, I'd either remove all others, or put them in a tank to themselves.
After spawning, I'd remove the male, and put him back with the group, to allow the female to raise the young unmolested.
In a large enough pond, removing the male was not needed.
Below, Gymno sp. Paso Pache with fry in a 300 gallon Rubbermaid pond.
