Skirting board filler chipping off

Are you trying to achieve a perfect seam or just looking to hide the gap?

You can fill it with paintable caulk/silicone and proceed with painting.

Yes, gloss paint needs to be scuffed up so that new paint can adhere properly unless you are applying an oil-based paint.

If you want to turn this painting project into a painful multi-day renovation then remove the skirting, sand or scrape down the protruding paint from both the wall and skirting, patch the wall with mud, sand, prime, re-attach the skirting, apply paintable caulk or silicone, and finally paint the skirting and wall.

It would be best to "rake" out the loose and jagged paint in the gap so that it can then be filled. After cleaning out the gap you should use a latex caulk to fill the gap. You may be able to find a color to match the paint you intend to use.

Inject the caulk along the gap,press it in with your finger and wipe gently with a dampened sponge until you're satisfied with how it looks.

I would suggest making sure the molding (trim) piece in the picture isn't loose and if it is to re-secure it with finish nails and such.

No need to de-gloss before painting, but a quick cleaning with TSP, ammonia, Windex will guarantee a successful paint job.

I would sand the board slightly to remove the gloss paint sheen. Then I would put masking tape or the newer type edging tape, green or blue, on both the wall and baseboard, just past where you want to fill. Clean out the loose debris and use a paintable silicone caulking to fill the slot. After you add the caulking run your finger along that caulking to give you a smooth finish. When the caulking has been smoothed, remove the tape to leave a nice straight edge. After the caulking has dried you can paint the baseboard as you like.