It was a strike that saw Pompey take the lead for the first time of the evening in Nailsworth.
Zak Swanson collected the ball from Abu Kamara and drilled the ball inside Jamie Searle’s near post early in the second half.
Kusini Yengi had earlier cancelled Tyrese Omotoye’s opener and later wrapped up Carabao Cup progress from the penalty spot.
And with the Blues making 10 changes at Forest Green, Swanson felt they produced an impressive display.
The right-back said: “It’s very pleasing after making so many changes and we got the result in the end, but it was a tough game. We grew well in the second half and took our chances.
“I was always thinking to go for the near post and it was a good pass from Abs. That’s what I want to do more often.
"That’s actually the first time a lot of us have played together and I thought that we did really well overall."
“I like getting forward and am looking to contribute more to the team, whether that’s with goals or assists.
“That’s actually the first time a lot of us have played together and I thought that we did really well overall.
“Kusini got another two and is class. He’s a big lad who holds the ball up so well and runs in behind. He’s a defender’s nightmare.
“There were a lot of new signings out there and after a good pre-season I think we’ve gelled together pretty well.”
Swanson impressed after not being named in the matchday squad for Saturday’s league draw against Bristol Rovers.
He said: “Of course it’s frustrating, but it’s the gaffer’s choice and you have to respect that. When I come into the team, I have to show that I should be there.
“We’ve got a strong squad and there were three good players left out on Saturday. It’s disappointing, but you just get behind the lads and wait for the opportunity.”