You certainly are challenging yourself trying to root Gardenias this time of year! The method described below is one I've had the most success with rooting softwood cuttings. Hardwood cuttings are best propagated by air-layering.
Take several tip (green)cuttings of about 5-6", stripped of their lower leaves and pot them together in a small pot with dampened sterile, peat-base potting mix.
Tent the cuttings in a plastic bag into which you've blown air, then seal and place the cuttings in a warm, bright location. Plastic tenting retains the humidity necessary for the leaves to survive until roots form as indicated by new plant
growth. To lessen the risk of fungal infections, ensure the plastic bag is large enough that the leaves don't touch the plastic once filled with air. Should condensation form within the tent, open the plastic long enough to lower the humidity, then reseal after refilling with air.
Check the soil regularly and water only if necessary. Soil should be damp but not soggy - it is unlikely you will have to water the cuttings again until the tenting is ready to be removed. Once new growth appears (about 5-8 weeks, perhaps longer this time of year), gradually remove the tenting over several days.
You are on the right track but it might be helpful to try the tenting. Patience is key and remember, Gardenias are not the easiest plants
to propagate, so don't be discouraged if you don't first succeed. Best of luck!