They give you a certificate number right away. Print the page or copy it down. You can book your flights using the cert number as soon as you finish your purchase. They will also send you an e-mail with the certificate number, but it doesn't arrive immediately. I don't believe you can use gift certificates to buy drinks onboard because the FA's have no way to know what you have. You can use them for anything online or by phone and unused balances never expire nor are you charged a service fee. You can use them to pay for anyone, not just yourself. Your A+ flight credits show up in your A+ account overnight (usually), but the site says to allow up to 7 days.
I don't think there is a limitation to not being able to use the GC for yourself. I "gave" one to my wife, she used it to pay fees for a rewards flight and then I used it to pay fees on my rewards flight. You can't use it during the online checkin process either. So purchasing upgrades online with a GC is not an option.