I too have problems with gnats and aphids. What I do is spray the outdoor plants with soapy water. Use somethng like Sunlight soap (not detergent). Spray the plants until soaked, it will not harm the plants and it smothers the aphids and they drop off.

In the house I soaked my plants in a sink of water with soap in it and lightly sprayed the leaves and stems. This seems to work for me.