I have a Glem Gas Oven that can take over 30 mins to heat to desired temperature. I was advised to place a pizza stone on the bottom of the oven but it still takes longer than 30 mins to reached desired heat. Any tips please.
I don't see anything unusual here. Ovens are a big mass of steel and other materials that has to spend some time heating before it is at working temperature. Many ovens turn off as preheated somewhat earlier - see also this question where the answers speak of 15-20 minutes of preheating on average - but this doesn't have to be the correct answer for your case. Maybe you are looking for a higher temperature, maybe your oven is larger, or maybe your oven only has one heater on the bottom as opposed to the two heaters common in electric ovens. Also, I suspect that many ovens, even though they turn their light off as "preheated" after 15 minutes, are not truly as well heated as they should be - if I bake several things back-to-back, the later ones bake quicker and more evenly.
A pizza stone will actually make the preheating slower, because you are adding cold mass that needs to be heated. So don't expect any change there, unless you decide that, by having a pizza stone, you can afford to place the food in the oven before it has reached its desired temperature, hoping that the heating help from the stone will compensate for the too-low-starting temperature.
If this is the case and the oven is working as intended, there is not really anything you can do about it. Baking is a slow method anyway, so you have to just take it as it is. You may consider adding a large-sized toaster oven (one of those with 30-40 cm width and depth of the cooking space) and bake quicker things in it. It will be especially useful if your main oven doesn't have a top heater.
If you think that the oven is designed to preheat quicker, then you have to call customer support or an independent repair service to see where the defect comes from.