Gabriel was turned in by his adoptive family when he was fifteen and manifested his powers (there was a lot of collateral damage that day). So he and Marcus are probably the old vets, given they've spent most of their lives in the Corps. He's learned not to form deep attachments, as people do get killed, but as Marcus is pretty much damage-proof, he's become attached to him. Gabriel is probably the amused one; in spite of everything, or possibly because of it, he's maintained his sense of humor, which is admittedly pretty macabre. He appreciates Marcus' sense of fine clothes and it's probably influenced him to some degree.