OxyGuanine in single-strand DNA

GROMACS version: 2021.4
GROMACS modification: Yes/No

Hi there. I want to make the topology of a single strand DNA but actually, it has been modified. In fact, there is a OxyGuanine in my single-strand DNA. I am wondering if any kind of software or approach exists to help me build my topology.
Here there is my sequence, all of the nucleotides are deoxy except the marked G.