I am not a lawyer, and UX/UI can absolutely be subject to copyright. This is a law question, and should be treated as such. From a legal perspective, the decision whether or not to move forward with a particular design that may infringe on copyright should be made by your legal team. If nothing changes, your goal should be to ensure that the design is run by your legal team before getting the go-ahead to proceed. Do this early in the process if possible. The later your legal team has objections, the more expensive it'll be to get them fixed.
That said, I think there's a discussion to be had here about a deeper, more philosophical approach to building new UX/UI.
Pablo Picasso once said "Good artists copy, great artists steal."
I'm not saying that you should steal Gmail's interface-- far from it. Picasso's quote makes the point that great art has some universal aspects to it, and many artists are just building on the work of each other to create something new and powerful.
UX isn't very different in this respect. One of the reasons GMail is so popular is because of its very good usability. Additionally, due to it ubiquity, many of its patterns are now embedded in our collective conscious as the standard in their relative areas.
Furthermore, if your app has anything to do with messaging, the chances are that the layout and functionality look very similar to other messaging apps, GMail included. Laptopmag.com had a great article a couple of years ago describing how various email competitors were trying to differentiate themselves frome one another-- a difficult task, given that they are all asking their users to perform the same function.
So while it's OK to come close to a Gmail-type layout, the real question to ask here may not be "Am I too close?" but rather "How can I make this design better to fit the needs of my user?" If you feel this design is the A1 standard, then I think your best approach is to explore how your company might be able to put its own shiny finish on the product. If it's not, maybe you need to re-visit your drawing board.