This is more the result.

Generally today an Ethernet frame, which is the basic atomic unit of information over the wire, is limited to 1500 bytes (the MTU, or Maximum Transmission Unit).

If you want to send more - the IP layer allows for 64k bytes per IP packet - you need to split the IP packet into multiple (64k / 1500 plus some header overhead) frames. This is called segmentation.

Before GSO the kernel would do that which takes buffering and CPU time to assemble the frame headers. GSO moves this to the ethernet hardware, which is essentially doing the same thing only hardware accelerated and without taking up a CPU core.