Gold Rush is a special event that happen in clash royale every new season , these seasons last about 30-40 days. These special Gold Rush events happen 1 time each season or 2 times some seasons.
In these Gold Rush events you can get gold by destroying enemy towers , you can get this special gold reward only if you play 1v1 mode.
Now the real question , “How Much Gold”
Well it depends on supercell they keep on changing the rewards , but in recent times we can say that the least we can get is 10K gold(Happens Frequently) and max is 250K gold (Happens Very Rare).
These special Gold Rush events are only available for a span of 3 days , once/twice each season.