URGENT! Are gift cards won from playing games taxable income?

I just googled it and found out they are taxable, I won a £20 voucher earlier in the year for playing games on an app while I was signing on and didn't declare it as income as I didn't know it counted I thought only actual money counted. There's nothing I can do about it now as I signed off a couple of months ago and doubt I could log into my account now, I was probably overpaid, I'm worried now what do I do?

Edit I've managed to log in to my universal credit account and the £20 voucher wouldn't have affected the amount I was paid, I had a loss that month as I had business expenses that were higher than what I earned even with the £20 voucher it was still a loss a smaller loss but still a loss as I still earned less than what the expenses were so I was paid the maximum amount I could have gotten. Do I still need to contact them? I can't through my account as my claim was closed, only claimants can contact them and they'll probably be closed for the Christmas/new year break but should I call them after?

(Caveat: I'm no expert on Universal Credit.)
I think perhaps you're confusing different concepts: whether you owe tax on the £20 voucher you received, and whether the receipt of a £20 voucher impacts your Universal Credit. These are different concepts with different rules.
I'm going to assume that as you've said that you "won a £20 voucher earlier in the year" that this was the result of gambling. Is that correct?
It's important for a couple of reasons:
(1) In the UK, the proceeds of gambling are not taxable (instead, the organisation providing the gambling services are taxed).
(2) Gambling proceeds do not meet the definition of earnings within Universal Credit Regulation 52 and are not taken into account as unearned income.
However, gambling proceeds are considered "capital" and, as I'm sure you know, your entitlement to Universal Credit reduces if you have capital (e.g. money, savings, investments) in excess of £6,000. Do you?
That's the only theoretical impact that I can see this £20 voucher having. And even then, you're entitlement reduces by £4.35 for each £250 you have in capital above £6,000. So if this £20 doesn't take you into the next £250 band, then it has zero impact anyway.

No it wasn't gambling, there are apps/websites such as that you can download/use and play video games in exchange for cash or vouchers/gift cards or other rewards, I earned £20 from playing video games through this app and when you cash out there's an option of getting a £20 Amazon voucher, £20 in cash sent to your PayPal and there was another £20 option. I chose the Amazon voucher because before that I made £20 and when I tried to cash out through PayPal I never received the cash and to this day I still haven't received it, it's disappeared from the app like it never happened and the reviews for that app have said they never received cash or got it many months later but the Amazon vouchers always work so I picked the Amazon voucher and it worked