What is the difference between ガスコンロ and ガスレンジ?
First I must say that the definition of Japanese word レンジ is a bit confused. In everyday speech, it's understood as an oven-like cooking device, or more practically, by nine out of ten chances it'd mean 電子レンジ "microwave oven". (As an aside, オーブン means non-electrical oven as opposed to this.) But it also has the same meaning as the English "range", that is a synonym of "cooking stove".
2 : a cooking stove that has an oven and a flat top with burners or heating elements (Merriam-Webster)
１ こんろと天火を備えた加熱調理器具。ガスレンジなど。「―フード」 (デジタル大辞泉)
(A heating device for cooking that equipped with burners and oven.)
The second definition still survives, mostly in device makers' catalogs, thus should never be forgotten.
On the other hand, コンロ (it has kanji, 焜炉) which is also a synonym of "cooking stove" nowadays, originally referred to those cooking furnaces on top of which you can put materials. In modern sense, you can say the cooktop with burners is コンロ, or each of the burner pit is, or the whole stove unit that has cooktop is.