In France, you will often find:

Tondeuse à gazon thermique

(short for "à moteur thermique")

See French Wikipedia.

In Switzerland and Canada you might find:

Tondeuse à gazon à essence.

I would say that both are correct. In fact, the second is more straightforward.

My guess is that the rather convoluted use of "thermique" was dictated by the double use of "à" in "à gazon à essence", which is intuitively disgraceful to the ear.

I have also seen:

Tondeuse à gazon essence,

where the second "à" has been dropped, which is business language, not regarded as grammatically standard. Which tends to reinforce the point that the French language dislikes such a repetition.

As @fralau said, in Canada we would say,

Tondeuse à gazon à essence.

We could also say,

Tondeuse à gazon motorisée.

However, in actual conversation, “Tondeuse à gazon” or even just “Tondeuse” are assumed to be referring to a gasoline lawn mower unless it is otherwise specified.