My house was built in Texas in 1959 and has 2 wire electrical circuits
in the flexible metal conduits. The only outlet in my bathroom is one
that is part of the wall lighting fixture over the medicine cabinet. I
would like to install a GFCI just to the left side of the medicine
cabinet.

The bedroom on the other side of the wall has an outlet below and a
couple of feet to the right of where I want to install the GFCI. My
plan is to cut out a large enought square of drywall (about 3 feet by 3
feet) from the bedroom to expose its outlet plus the area where I will
be installing the GFCI in the bathroom. Then I'll run about a 6'
length of 12/2 wiring with ground from that outlet through one stud and
up to power the new GFCI, and ground the boxes together with the ground
wire. I would rather run the wire from the bedroom outlet than from
the bathroom wall lighting fixture because the that way wallboard
repairs will be hidden behind some furniture. If I have to use the
light fixture the wallboard repairs will be more obvious.

Does this sound like a workable plan, and can I use a 15 Amp GFCI since
it's an older house? Will this wiring addition cause any problems when
and if I decide to sell the house? In other words, will they say that
since I've added one GFCI I now have to bring the entire wiring system
up to code?