I haven't got any experience of this but I have just looked it up myself and it appears that it can be a sign of some neurological disorder, but that many children grow out of it. However if it has become a habit to them, it can take a while for them to grow out of it, and there is a potential that it may cause their hamstring to shorten if it continues for too long.
There were numerous references to exercises that you can do with your child to help them stop walking in duch a way if it has just become a habit. As your DC is only walking like that for about 25% of the time it shows that they can walk normally - without it hurting them - so I don't think you should mention it to your child at all.
I wondered if you could make up a 'game dance' with them that you try to fit into their day most days, for about 5 minutes or so. Put some music on that your child really likes, and make up a heel led dance, so maybe you could do a few steps that start on your heels then roll forward on to your feet, then keeping your feet flat, bow to your partner (your DC obviously), and then they have to do what you did. It is by necessity a very simple little dance that maybe you and your DC can enhance as you go along.
Also, if you could go on some short walks outside with them, and once you are walking along you could suddenly say " oh, shall we practice the walking bit of our play dance?", and then you both do some heel led steps, then make them into a couple of heel led strides, then back to normal, and then do some tiny weeny heel led steps, and then you both finish your walk walking as you normally do, whilst maybe counting how many chimneys you can see, or satellite dishes, or birds on roof tops, anything so that your DC doesn't become obsessed with how they are walking.
I am sure that you can come up with much better ideas, or look some up on the net. After gently encouraging your child for a few weeks to walk 'normally' more frequently, you will hopefully be in a better position to judge your child's walking. If you are still worried go back to the GP, without your DC if possible, and ask them to refer your child to a paediatrician, and then take it from there.
Good Luck Op, and as your little one doesn't walk on their tip toes all of the time, and that you don't have any other developmental worries to do with them, I don't think you need to worry too much. 🌻🌻🌻