Are one armed pushups a significant way to gain muscle?
No. It's just a show-off move....totally unnecessary and I personally think its an injury risk.
Stick to the big compound moves like squat, bench, overhead press, pull-ups, dead-lifts. You will gain muscle.
If you have no access to the gym or any equipment and you are looking to hit the chest, then stick to normal push-ups and add weight to your body somehow to increase the resistance - or just do them really slowly.
However, if you REALLY want to gain significant muscle - you need access to basic gym equipment - bench, bar, plates, power-rack.