It depends entirely on the material the christening gown is made of whether anything at all can be done to lessen the discoulouring.
If it is silk - please do not try anything yourself, but ask the advice of a dry-cleaner used to dealing with natural silk.
If it is 100% cotton, either batiste or lawn, or some other fine cotton, it can stand being washed in fairly warm water, but I would personally not risk bleach on a fabric that is, I assume, at least 20 years old now.
My grandmother bleached fine fabrics, including lace, by washing them in plenty of lukewarm water, rinsing them well and hanging them out to dry, spread over a towel and with a cloth between the pegs and the garment in frosty weather.
Now frost is not very likely right now, so try half-drying the gown then wrapping it in a white towel and putting it in the freezer for an hour or so.
If it is poly-cotton or some synthetic fibre none of this will work and may well do harm, but you could try steeping the gown in buttermilk, overnight then rinsing it thoroughly and re-washing it. This was said to work with nylon that had turned greyish.