The Bachelor star spoke with ET at the Critics' Choice Real TV Awards in Beverly Hills on Sunday, and he opened up about Gates' recent romantic revelation that she'd accepted her longtime boyfriend's proposal.
Underwood said he was "not at all" surprised by the development, explaining that even when they were both on Bachelor in Paradise, where they first met and started dating two years ago, "You could tell those two love each other and have an appreciation for each other."
Gates announced the news to Instagram on Sunday, with a black-and-white snapshot of the happy couple, which she captioned, "I’m keeping you forever and for always, We will be together all of our days, Wanna wake up every morning to your sweet face…"
"I'm very, very excited for them and congratulations to Adam and Raven," Underwood shared, adding that he's not sure if he'll get an invite to their big wedding, but he'd go if they offer.
As for his own relationship with Cassie Randolph, whom he sparked a romance with following the last, dramatic season of The Bachelor, Underwood said his ladylove wasn't able to come to the star-studded awards show with him because, "She's getting ready. She starts her internship down in Newport tomorrow so she'll be doing that all summer long."
"Her schooling is really important to her so I'm there to support her right now through her schooling," he added. "She has two more years left on her speech pathology degree so she has that going on, so were sort of just taking things in stride."
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'The Bachelor's Colton Underwood and Cassie Randolph Share Engagement Update (Exclusive)
The reality star couple -- who bucked the Bachelor trend when they decided to give their relationship a chance without getting engaged by the end of the season -- addressed the possibility of getting even more serious in the near future when talking with ET's Nancy O'Dell at the ACM Awards back in April.
"I think, right now, at this point in our relationship, we’re enjoying this, trying to be as normal as possible," Underwood explained. "But you know, we’ve talked about our engagement and talked about taking the next step in our relationship and [decided] one thing at a time."
Check out the video below to hear more.
The Critics' Choice Real TV Awards airs June 9 on VH1.