Old gloss is shiny, hard and non-porous, which is exactly why fresh paint won't stick to it on its own. The job is to give the new coat something to grab onto. Three steps, no shortcuts.
First, key it. Sand the whole surface with around 120–180 grit until the shine is gone and it feels dull and slightly rough. You're not stripping it back to bare wood — just breaking the glaze. If the old gloss is solvent-based (likely if it's more than a few years old) and you want to know, a rag with a bit of meths will lift modern water-based paint but not old oil gloss.
Second, clean it. Sanding leaves dust, and old woodwork carries grease — especially skirtings, kitchen trim and anything near hands. Wash down with sugar soap, rinse, and let it dry properly. Paint over grease and it'll fish-eye or peel no matter how well you've sanded.
Third, prime. This is the step people skip and regret. For glossy or dodgy surfaces I'd reach for Zinsser BIN (shellac-based, grips almost anything, dries fast) or Zinsser Cover Stain. Both give your topcoat a sound, matt base to bond to.
Then topcoat. For modern water-based trim that won't yellow, Little Greene Intelligent Eggshell is a cracking choice and tough as old boots. Farrow & Ball Modern Eggshell is the other strong option. If you want a classic just-off-white on the woodwork, Pointing or Slipper Satin both sit beautifully against most walls; Wevet if you're after something cleaner and cooler.
The "but what about" question: can you skip sanding with a bonding primer? On sound, well-keyed gloss, BIN does grip remarkably well — but I still wouldn't trust it on a glassy, untouched surface. A light sand costs you twenty minutes and saves you redoing the lot. Don't gamble it.