Not trying to skirt the system by any means, I simply forgot I was getting a 1099-B until now and had already completed my taxes in early Jan.
Can I claim this gain next year instead? Ultimately, I am not even sure it would have had any effect as I received a decent refund.
You cannot report the gain in a different year from the year you receive it.
There is no duty to amend a tax return if you made an honest mistake (even if the mistake results in more tax owed). However, you may wish to file an amended return, either to get a refund or to stop interest/penalties from accruing. You might want to enter the cap gain and see if it makes a difference. It might not result in any extra tax (some people are in the zero percent cap gain bracket).
More detail here:
https://www.thetaxadviser.com/issues/2017/jan/amed-return-client-decision.html
"The Supreme Court held that, despite these and other references to amended returns in the Treasury regulations, "[n]one of these provisions, however, requires the filing of [an amended return]" (Badaracco at 397). Similarly, the Tax Court held that taxpayers are not required by statute to file an amended return when an originally filed tax return is discovered to be incorrect and that the failure to file an amended return in this context does not, by itself, establish an intent to evade tax (Broadhead, T.C. Memo. 1955-328)."
meaning if the overall net result is the same (after adding this in), that it's not necessary to file an amendment?
It is not required that you file an amendment period. You might want to do so to avoid possible interest and penalties should the IRS discover your error.
If there is no additional tax that would be owed, there would be no interest and no penalties.
You have to claim your income in the year it is earned.
If you do not amend, you may receive a letter from the IRS. Even though the gain may be only $84, they use the gross amount only, so you could receive a bill for several hundred dollars, depending on how much your investment sold for.