I just thought I'd post my experience with my upper endoscopy/gastroscopy today, as I'd been putting it off for ages due to the awful stories I read online. I think people are more inclined to post their experience if it's an unsually traumatic one, so I'd like to add another good story to balance it out!
Background: I'm a 26 year old female. It was about a year ago that my GP pushed for a gastroscopy, but after doing my research, I refused. Even the thought of it made my heart skip a beat. I was in the hospital once for an ultrasound and walked by the endoscopy unit... I remember thinking, no way am I ever going in there! Then this year I suffered a few bad bouts of vomiting, and my GP again encouraged me to have a gastroscopy. By then I was so sick that I just wanted to do whatever I could to help myself.
So, in preparation, I read as many encouraging experiences as I could. By the time I arrived at the hospital today (University College London), I was in good spirits and excited to finally get it over with. I knew I was going to opt for just the throat spray and no sedation, as I always feel like my best chance of getting through anything is mind power.
After an hour's wait, I was led into the procedure room and prepared by two lovely nurses. The doctor sprayed the numbing stuff into my mouth (it kind of stung and tasted weird, but it didn't bother me). I was expecting to feel uncomfortable as my throat was numbed, but it wasn't bad at all. I could still sort of feel myself swallow. If you feel like you can't swallow and it weirds you out, just put your hand on your throat and feel yourself swallow from the outside.
The nurses then inserted the mouth piece, and the doctor told me to close my eyes. I felt the endoscope go to the back of my throat. He told me to swallow, and next thing you know I felt it (very faintly) go down my throat. I could breathe normally, as the endoscope was a lot narrower than my esophagus (I had pictured it as being so big it would block the air!), and I could even swallow without choking. Swallowing had been my main concern, because I find it hard not to swallow for long periods. But I was able to breathe and swallow as normal.
While the endoscope was in my stomach, I was comfortable enough - I could have lasted like that for another hour. A few minutes in, the doctor moved the endoscope into my duodenum, and this is when it felt a tiny bit sore for a few seconds. It was more like discomfort than pain. He took some biopsies, which didn't hurt at all, I just didn't like the general feeling of something in that part of my intestine. But that part was so short (20 seconds or so) and the doctor reassured me by saying the procedure was almost over. Next thing I know, he pulled it back into my stomach, then all the way back up my throat. The whole thing lasted about 5 minutes max, no gagging at all. I would do it again right away if I had to! I'm really glad I didn't opt for sedation, as being aware of how easy it was going is what kept me calm throughout. I was out of the hospital and on the bus home within 15min of being done.
Here's to reassure you about some common fears people have:
Gagging and being sick: The throat spray will do a lot to stop gagging, but even if you do gag, remind yourself that it's okay. It doesn't feel nice, but you're not going to die or have permanent damage, and there's probably nothing in your stomach for you to vomit up. I've had plenty of nights vomiting continuously for 10 hours, thinking I was going to suffocate, bursting blood vessels in my face from the violent gagging. So, absolute worst case scenario, I'm sure you can manage a few minutes of that surrounded by nurses that are taking care of you. 
But you probably won't be near that bad!
In preparation, I actually trained my gag reflex to be less sensitive. Touching your gag reflex over time will desensitise it. If gagging really has you worried, I suggest Googling instructions (sword swallowers do this, as do people who... ahem... deep throat). This could be especially useful if you're the kind of person who gags when they brush their teeth or swallow pills. Sensitivity should return when you stop "exercising" the reflex (it's not good to weaken it forever, as it's there to stop you from choking!). Making a fist with your left hand and squeezing your left thumb inside it is also said to suppress your gag reflex.
Not being able to breathe: Nothing to be afraid of here, there is plenty of space in your throat and nose to breathe.
Not being able to swallow: I managed to swallow just fine throughout, but you may not even need to, as the nurses suck out excess saliva with one of those dentist tubes.
Pain: Many people experience no pain, others just a tiny bit of soreness or discomfort at points. But probably nothing worse than the symptoms that have led you to do this test.
Panicking: If you are vaguely comfortable with things going past your gag reflex (as I was after desensitising it), you shouldn't have anything to worry about. I could hardly feel the tube in my throat. Thank goodness for that spray stuff! And just keep telling yourself it's a very short, safe, painless procedure, and each second you bear is one second closer to it being over. In the lead up to test, read other people's good experiences, and focus on the positive of how glad you'll be to have finally done it.
If you have any questions, I'll be happy to answer. If you're still really scared about having a gastroscopy, just remember, I was exactly like you a few months ago! And I managed to change my mindset to the point where I was laughing and joking with the nurses as they prepared me. 