Presumably, G would win this due to size and power.
On the other hand, SP is quite nimble despite his great age. The Matrix rejuvenated him when it revived him. He could dance around G and slice him up steadily. It could be like the Monty Python battle between King Arthur and the Black Knight.
No, more seriously SP would ram that sword through some vital organs and kill G as quickly as possible. SP is pretty motivated in DotM. He's driven by the need to save Cybertron. He wants to use Earth's energy to repower Cybertron and thus have it repair itself. If SP maintains this level of motivation, victory over G is quite plausible.
Anyway, I'm giving this to G but one wrong move could turn the tide. If SP finds himself losing, he would retreat swiftly and then plot revenge.