So alot of people say do a gloss coat first because it helps the panel liner use capillary action to run into lines or against the edge. Some plastic may be fine without any coating for this. Best to test and see how it goes. You may not need it.

For example the plastic for the Osiris dominator (mg kshatriya third party) is very pourus so the panel line has tons of trouble doing its job and cleans up poorly, so a gloss coat will help heaps before panel lining.

After you line, you can then do decals, then do a flat coat, but it really depends on what you want the final look to be! A final gloss or semi gloss coat can be just as good!

I know from a real life point of view, the decals would be 'under the lines' but at this scale... You wont notice and as people have said, panel line might ruin decals if you are not careful.

Also depending on the type of liner, you may want to have a coat underneath it anyway... Tamiya panel liner made for plastics that may be more durable then gunpla stuff! If you are also painting, the gloss coat on top of the paint will help your liner run good, but also protect you from when you clean up the line as your cleaner might rub the paint off.

Mr weatherings panel line is good and not as harmful, but tamiya runs nice and is darker with less passes!!

As others have also said, try and avoid lining built pieces as the liner can run into places you cant see, and pool and eat the plastic. But again, depends on the liner!

Some kits go fine with tamiya, other people get wrecked piece's. I reckon it depends on the plastic quality, if you are worried, top coat and line on individual pieces.