Gas heat in my apartment is not keeping me warm at night, even though air temperature is 76 degrees. My apartment management provided a separate insrtument to measure temperature and it reads 76 degrees at nght. Any reason why?

Wait while I test your 'separate instrument' against my thermostat... hmm.. don't see anything.
Of course we can't know why!
Please start over, and describe your regular wall thermostat brand and style, and this instrument, and what it really is.
Possibly your landlord is pulling a fast one.
No law anywhere requires 76 degrees though.
The easiest thing to do would be to go buy a thermometer for about $10.

76 and you aren't warm? That is a pretty high room temp.

Call your landlord and complain but 76 is plenty comfortable it seems to me. Have no clue what this special instrument is so ask him about that too.

Could it be a thermostat? Does it look like this?

https://tse1.mm.bing.net/th?id=HN.607989991778091062&w=183&h=182&c=7&rs=1&qlt=90&o=4&cb=10&pid=1.7

Or this?

https://tse1.mm.bing.net/th?id=HN.608040290148878782&w=96&h=105&c=7&rs=1&qlt=90&cb=10&pid=3.1&rm=2

Maybe you just don't know how to turn it up. Ask the landlord to show you.

Consider yourself lucky they are willing to allow it to be that warm. Most places they are only legally required to raise it to 68 degrees. Being someone who has lived in a number of apartments, condo's, townhouses and single family houses over the last 53 years... may I suggest warmer clothes and an extra blanket on your bed.

I don't even heat my own house to 76 even though I would be much more comfortable because it costs significantly more every month for that extra 6 or 7 degrees.

Also dry air feels colder than humid air at any given temperature. But getting a humidifier for your own comfort is your expense to pay for...not the landlords.