I have been using Rinnai Gas Dryer RD-600 CG from City Gas to dry my clothes for almost 8 years and have only done one time of repair. It is a durable machine and can dry clothes say 5 to 6 kg of washed clothes. The time taken to dry one load is about 30 minutes. Normally, for an electric dryer, the time taken will be double of the gas dryer.
When you start the cycle, the gas will be ignited to provide the hot air to dry your clothes. You may want to provide some ducting to channel the hot air out of the house. It is recommended to install this near a window for ventilation. This Rinnai Gas Dryer can be installed above your washer via mounting brackets to the wall. I don't think it can be stacked onto the washer directly as it is 5cm wider.
I have anticipated to install this Rinnai Gas Dryer in my new condominium unit but now after collecting the keys to my unit, I realised that the width of the washing area is only 64cm. Even if it is 65cm, I don't think I will be able to fit the Rinnai Gas Dryer into the space because there should be some clearances between the wall to the sides of the dryer. I was very disappointed with the design as the developer has not considered that people may want to install gas dryer which is around 65cm in width. I was hoping that the washing area width will be around 70cm which is good to fit in the gas dryer. I was considering the possibility of knocking down the wall to make way for the gas dryer but the DB, OpenNet Fibre Optic cable and telecom points are located behind the wall. It will be a major renovation work if I need to hack the wall.
There are only two options available. One is to get an electric dryer instead and stack it above the washer. The other option is pray that Rinnai can manufacture a gas dryer which is around 60cm in width (which is closer to the standard washer/dryer dimension). The later option does not seem to happen in the next couple of months or even years. Well, I will need to switch to an electric dryer for my washed clothes. This will definitely increase my electricity bills.