Some of those questions depend on how large the spotted gar are, and if they are actually spotted gar. I've kept shortnose and florida gar in the past. I kept the shortnose in a 14'L x 6'W x 4'D outdoor pond year round (in Kentucky) and they did great. Indoors I kept the spotted at a smaller size and the florida gar in a 125gal. I don't think it was large enough for a long term home. Eventually I moved them to a 72"L x 30"W x 24"T 220gal tank, and even that seemed a little cramped after a while.
I would probably suggest the smallest aquarium to be at least 2 feet front to back and 6 feet long. An indoor pond would be better than a tank, I would go for something round or oval to eliminate corners, and at least 6 feet across.
As far as tankmates, anything large enough not to become a meal, and avoid overly aggressive cichlids that might attack them. I kept with peacock bass, red bay snook, datnoids, vieja synspilum, larger geophagus, and motoro stingrays and various less aggressive south and central american cichlids and catfish without any issues.
Standard water chemistry should be fine. They are natives and so a more temperate setup might be appreciated, but mine did fine at tropical (78-80F) temps.
Cost will depend on where you live and what you decide to feed. Mine started on live food (feeder minnows and earthworms), but over time I got them on to freeze-thawed shrimp and silversides, and eventually onto pellets (massivore).
