This indicates an attack attempt against an Integer Overflow vulnerability in Libgd.
The vulnerability is caused by an integer overflow in GD Graphics Library which leads to a heap overflow. By sending a specially crafted G2 file to the target, a remote attacker could overflow a buffer and execute arbitrary code on a vulnerable system.
GD Library libgd prior to 2.2.3
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.