It's possible for water to seep into leaves if the succulent is in direct sunlight. When using leaves, make sure to obtain the whole leaf. The rest of the pot should be taken with you. Fungus gnats are attracted to moist soil containing lots of plant matter and leaves that have been decayed. Keep your pot dry and clean. Soil plays an important part. That's why you need to ensure that your soil is thoroughly wetted. The water won't penetrate beyond the top of the soil if you only use a small amount of it.