Alternator vuage.

Amp gauge or Volt gauge?
Could be 2 things that have the same symptoms.
Amp gauges are sometimes finicky and won’t cut in and register till engine revs up a bit off idle rpm. Depending on how Volt gauge is wired thru key or not it may read battery power full time with key off or only when key is on. They get lazy sometimes and rise slowly.
If alternator is slow to cut in or needs a slight rise in rpm to begin charging which is pretty normal with age there may not be anything wrong with gauges at all.

Lazy gauges or lazy when cold alternator= same symptoms with gauges but not necessarily anything that needs fixing if everything is charging and keeping batteries happy.