There are $2$ cases:
1. $n$ and $m$, both are even $\rightarrow (aa)$*$(bb)$*
2. $n$ and $m$, both are odd $\rightarrow a(aa)$*$b(bb)$*
Regular expression: $\rightarrow (aa)$*$(bb)$* $+$ $a(aa)$*$b(bb)$*
Regular grammer:
$S \rightarrow aaA/aA_{1}/\epsilon$
$A \rightarrow aaA/bbB/\epsilon$
$B \rightarrow bbB/\epsilon$
$A_{1} \rightarrow aaA/bB$