20 gallon would be nice but it's not absolutely necessary for housing just one gecko. For 1 single solitary leopard gecko a 10 gallon will suffice. The single most important part of gecko husbandry aside from enclosure size are nutritious feeder items, making sure that the insects you are feeding your geckos are properly gutloaded with nutritious feed 12-24 hours before feeding them off to your geckos (I gutload my roaches with unmedicated chicken feed, it's cheap and very nutritious). Calcium with D3 dustings once a week and calcium withOUT D3 every other feeding. I also feed my geckos roaches with a dusting of multivitamin powder once a month.
*edit, also forgot to add, temperature is extremely vital as well, if your temps aren't high it's likely the gecko will have trouble digesting their food, or not want to eat. I like to keep my Leo's warm side @ 90 - 92 and cool side around 75 - 80, depending on the season.