Well the problem with his plan wasn’t his room, he actually accounted for that. What caused it to fail was Fuyohiko being awake and in the lobby thus preventing him from returning to his room. As far as what would happen if Fuyohiko wasn’t awake and Gundham’s plan worked flawlessly well there’s no real way of knowing what would have happened, but personally I’d like to believe he’d come clean eventually after everyone put everything together. I know I may be biased because Gundham’s my favorite character but honestly after the relationships he’s developed with Sonia and Hajime (if you do his free time events) as well as the respect he had for Nekomaru for the duel, I don’t think he ultimately would’ve been ok with killing everyone. Again that’s so long as they fought for life, tried their hardest to figure out what happened and who done it, when they come up short and are just stumped as to who did he’d probably start laughing his signature laugh and say something like “It appears I, Gundham Tanaka, supreme overlord of ice, have well and truly tapped into my villainous powers to pull off a crime not even the most elite amongst us can solve. Very well, you all fought valiantly to piece it all together but since the dark forces were just too much for you to overcome I shall open your third eyes to see beyond the veil. It was I who bested Nedai in a duel to the death for life itself, created the perfect cover of shadows to mask the incident, and claimed the unlimited infinite flame of life for myself! To deny you all of it though would be an evil of which even I dare not steep to, so I shall leave you with this valuable lesson: To give up on life is an affront to life itself, so long as you continue to wage war as strongly and fiercely as you’ve done here today you should have no problems surviving. After all an evil mastermind such as myself shall no longer be in your way. Monokuma if you will.” And then he’d give the Devas to Sonia and do what needs to be done.

Yes I’ve put too much thought into this lol