it gives 0.028 bitcoins, but when I've imported into electrum I see 20.818 mBTC
Is there a typo somewhere? I assume it's either 0.028018 Bitcoin, or 28.18 mBTC.
If you want to sell your Bitcoins, your options depend on the country you live in. Some are easier than others.
Since you have 'just' imported the private key, you might consider creating a new HD wallet (which gives you a 12 word seed phrase as a backup) and send your funds over there (sweeping).
This way your old private key will be worthless, and your new wallet (backed up by 12 words) will hold your balance.
Doing this also makes you the only owner of the private key. As it was a gift, the previous owner may still have a copy.
If the wallet was created before August 2017, you'll also own just over $10 in
Forkcoins, so don't throw your paper wallet away yet after transfering your Bitcoins.
mBTC stands for milli-BTC. So 1 mBTC = 1/1000 BTC or 1 BTC = 1000 mBTC.
I'm always surprised how many people don't know
metric prefixes. I guess that's the result of using Imperial units.