Thank you for your question and welcome to Just Answer.
You have two conditions in your mouth that are compounding upon each other to create these symptoms.
The first is that you are mouth breathing at night. Something in your nasal passages is preventing you from breathing through your nose at night. This is why you wake up with a dry throat. In addition, the gum tissues that you describe as red and inflamed are drying out at night. These are tissues that are supposed to remain moist and when they dessicate at night, they become irritated and will bleed easily, even when you sleep. The blood dries in your mouth and when you wake, it mixes with your saliva giving you the impression that there is blood in the saliva.
The mild gingivitis you have is the other factor causing this. Good oral hygiene will minimize this but won't go completely away until you can start breathing through your nose at night.
You should make an appointment with your physician or ear, nose and throat specialist to get to the bottom of this condition. Until you can get your nasal passages cleared up, you should try putting some Vaseline petroleum jelly on your gums and lips before going to bed at night to keep them from drying out. This should help to drastically improve the redness, bleeding and the flat line you observe, which is just dry skin.
I hope this answers your question.
If you have any concerns regarding this issue that I have not addressed, please let me know and I will get back to you with more information.
Sincerely, XXXXX XXXXX DDS