Imagine there are two observers $A$ and $B$ and a particle $P$.
$A$ and $B$ are at about the same point, $P$ is some distance away.
From $A$'s point of view, $P$ has velocity $V$ and $B$ has velocity $-V$.
P----> A <----B
Suppose $A$ and $B$ have the same rest mass $m$ and $P$ has rest mass $m_P$.
It seems to me that $A$ will observe a gravitational force between $P$ and $A$ of $F_a$ and between $P$ and $B$ of $F_b$ with $F_a\neq F_b$ because the relativistic masses of $A$ and $B$ are different.
However, B will observe forces $F_a'$ and $F_b'$ and it seems to me that $F_a' \neq F_a$ and $F_b' \neq F_b$ because $A$ and $B$ observe different relativistic masses for everything involved.
I've tried demonstrating $F_a=F_a'$ on paper and gotten into a tangle. Intuitively, I think this equality is false and expect the difference to be resolved elsewhere.
I have a qualitative understanding of general relativity, but can't handle tensors etc. Can this be explained with high-school maths?