I'm not sure if it's OK for me to post here because this is my first post and because of the sort of person I am, but I think I can offer a piece of advice about gyno and gyno surgery due to my perspective. Apologies in advance if I'm not welcome here.
I'm FtM (female-to-male) transgender and, as far as I know, if your breast tissue has fully developed under the action of estrogen, you can't "regrow" female-like breasts after having gyno surgery. The only way that a breast-like appearance could resurface is if you weren't fully developed prior to surgery and continue using estrogen, which could cause growth again because your breast tissue wasn't finished developing. A secondary possibility is that higher levels of estrogen can cause fatty tissue to accumulate in your chest, which could result in "man boobs", but that wouldn't be true breast tissue and could in most cases shed with sufficient diet and exercise, and of course, higher levels of androgens.
So, surgery is a permanent solution to any further true breast growth as long as you've fully developed before having surgery.
As far as having "hollows" after surgery, that would depend on your surgeon and your particular situation, but transgender men don't have "hollows" after surgery. We have fairly normal looking chests after surgery. I say fairly because obviously they will not be 100% scar-free, or look 100% like a cisgender man who has never developed gyno, but they aren't hollow. This is most likely because the surgeon, as others have mentioned, doesn't remove the entire gland, but because our chests (FtM transmen) have almost invariably already been exposed to estrogens long enough to have fully developed, they aren't capable of growing again after surgery.
So, if you desire a more cosmetically pleasing appearance, but also don't want to face the risk of further breast tissue development, I believe it would be best to wait until the breast tissue has fully grown, because as undesirable as that is, it would at least be temporary and, in my opinion, the long-term results would be more pleasing.
Obviously, none of this is to be taken lightly. Surgery isn't without risks and it should go without saying that if a guy isn't comfortable having a less than perfect chest, estrogens are the last thing he should be using, but if one is determined to go that route, I think it's good to have information available to help make the best decisions. That's why I felt inclined to contribute here, as I don't believe there's ever been a transman's perspective on this issue.
Again, if you've developed gyno, your chest will never be the same again and surgery will not restore your chest perfectly to its pre-estrogen state. Your nipples will still be larger and/or positioned differently. If your gyno develops into large breasts, you may even have to undergo a more scarring procedure, which leaves long, horizontal scars below each "breast". I believe it might be helpful for anyone considering or undergoing surgery to read about FtM top surgery, as it would actually be the same situation.
I hope this information can be of some use to someone out there. If anyone has any questions which I might be able to answer due to my different background, I'd be happy to offer what knowledge I can.