Former Cardiff and Tottenham full-back Gunter joins from Nottingham Forest for an undisclosed fee, although Reading say they have made a "sizeable outlay" for the 22-year-old.
Defender Mariappa arrives from Watford with the Royals stating they have paid "a significant fee" for his services.
"Having spoken to the manager, [director of football] Nick Hammond and now the chairman today, everyone has made me feel really welcome and I'm really looking forward to this opportunity," Gunter said.
"If you form a team spirit, a togetherness, you'll always do well in football - and if you work hard you'll get your rewards. Reading have certainly done that in the last few years. Hopefully that can carry on. Right now, I need to focus on impressing everyone at this club and I'm really excited to get going."
Gunter becomes the Royals' sixth summer signing, following the additions of Mariappa, Pavel Pogrebnyak, Nicky Shorey, Danny Guthrie and former Forest team-mate Garath McCleary.
Reading manager Brian McDermott is pleased with his latest acquisition and believes he epitomises the good work the club have done since promotion.
"I'm absolutely delighted that Chris has signed," he said. "He was very, very keen to come to us and is another player who is young and very hungry. He's already played more than 200 games at club level, plus another 37 for his country, which is phenomenal for someone so young."
McDermott has also backed Jamaica international Mariappa to shine at the Madejski Stadium.
"I'm delighted to get the deal to sign Aidy over the line, we have been chasing him all summer," he said. "He's been a very important player for Watford over a number of years, he is well known to most of our players already and also the staff here. Every single person I have spoken to about him has nothing but good things to say about him."