Bill wrote:

I was told by a co-worker that a new Part 91-mandatory compliance reg
has come out, requiring all airplanes to meet a min. climb gradient of
2.5% for second stage climb gradient, unless published otherwise. In
other words, if no specific gradient is published for XYZ airport,
then the airplane must be able to climb out at 2.5% minimum.

I am unable to locate any regs or anything that address this, so if
anyone has further info and the reg, AC circular, etc, please let me
know

It's not second-stage. That is a Part 25 performance requirement. Having
said that, your friend is about right. There was a big NPRM a few months
ago with lots of instrument procedures issues in it. One of the proposals
was to make obstacle DPs mandatory for all IFR operations unless an ATC DP
or radar vector is assigned. It can be argued that it has always been the
case that any climb gradient on an ATC DP becomes regulatory if you accept
the departure. Pretty much the same can be said for departure vectors.

If the rule is adopted as proposed, presumably the obstacle DP will gain
the same status as an ATC DP. If so, any stated CG will be regulatory.
And, if no CG is stated, then 200 feet per mile, not 2.5%, would be
regulatory.

When, or if, the proposed rule will be adopted is something I don't know.