I guess what I really need to know is whether there is any significant difference between deep cycle gel cells and automotive (starter) batteries except for amp hours.
Automotive batteries can deliver a large current for a short time, but dies quickly if you try to use them for deep cycling (i.e. charge, then discharge substantially, then recharge, etc.).
Deep cycle batteries have much thicker plates, so cannot deliver as huge currents (although they can still wreak havoc if shorted) as their automotive counterparts, but are much less affected by deep cycling.
They should not, repeat NOT, be used for the "opposite" purpose.
I have run a wheel chair motor at 12V in a 'bot (www.CupSucker.dk
) and with me on top, it would still run faster than I considered safe after it ran me into a wall (controlled with a loosely wired potentiometer for steering).
How fast do you want your lawn mover to go?