The 23-year-old has repaid the faith of Saints manager Ralph Hasenhuttl by keeping three clean sheets in six Premier League games to help push the club away from relegation danger.
Gunn was initially forced to be patient following a £13.5million summer switch from Manchester City and did not make his top-flight debut until January's goalless draw at Chelsea.
The more experienced McCarthy was then recalled for five games but Gunn has since won back the gloves and, following Saturday's crucial 1-0 success at south coast rivals Brighton, has no intention of going back to warming the substitutes' bench.
"It was tough (being dropped after the Chelsea game) but I had that taste then so I had the focus and I knew I could do it," said the England Under-21 international.
"I always believed in myself and thankfully it didn't take too long to come back around. I'm just happy that we're picking up results now and hopefully we can finish on a high.
"I've played the last four or five games in the league and picked up a good amount of points and I feel quite settled.
"I'm more than confident to keep my place and hopefully we can keep keeping clean sheets and winning games."
A solitary goal from captain Pierre-Emile Hojbjerg was enough for Southampton to win at the Amex Stadium and join the Seagulls on 33 points, five above the drop zone ahead of Friday's visit of title challengers Liverpool.
Gunn, who was called into Gareth Southgate's England squad while on loan at Norwich last season without being capped, believes two more wins should be enough to see Saints safe.
"It was definitely a six-pointer and I think if they (Brighton) had got the win then they probably would have been out of it, but we've dragged another team back in thankfully," said Gunn.
"We've just got to keep focused and try and get at least two more wins before the end of the season.
"Hopefully we can push on now and not linger around the bottom for the last few games. We've definitely got potential to do that.
"We've got a big game against Liverpool on Friday and I think if we pull a surprise out everyone will be hopefully forgetting about us for the relegation battle."