In my Latin-Italian dictionary I found this expression:
arva sanguineo gyro scribo
that is translated as:
I draw a blood circle on the ground.
But, gyro is an ablative. Why is it considered as an accusative?
The Latin you provide is actually incomplete (though not incomprehensible). The English translation is not accurate at all.
hasta subit velox equitis femur inter equique ilia,
letum utrique volens; sed plaga sedentis
laxato vitata genu, tamen inrita voti
cuspis in obliquis invenit volnera costis.
it praeceps sonipes strictae contemptor habenae
arvaque sanguineo scribit rutilantia gyro. (XI.509-514)
A literal translation of the sad spectacle:
The swift spear approaches between the horseman's thigh and the horse's flank,
wishing the death of both; extending his knee,
the rider avoided the blow, but the tip, missing its desired mark,
wounds [the horse] on its side ribs.
[The horse] goes headlong and loud-hoofed, disregarding the tightened reins,
and imprints the red ground with its bloody course.
A less literal translation of this last line that captures the meaning better:
...and stains the ground red with its bloody course.
Two notes about the last line that may be helpful:
Concerning the motivating question, the ablative is not functioning as an accusative. rutilantia arva is the object of scribo, and sanguineo gyro is the ablative of means, showing what actually caused the red ground to be imprinted in this way.