Replacing stacked gas washer/dryer igniter
For most every dryer you will need to remove panels and the drum to access the burner assembly. For stacked units, you should go ahead and "unstack" it; that is, remove the top dryer from the washer. Then you would normally open the door and look at the upper door-seal area to remove a couple screws that hold down the top. Remove the top to access the screws holding on the front panel. Remove the front panel and the drum to access internal parts.

If you have not done this before you should do more research or hire someone, as there are specific steps for removing and reinstalling the drive belt and drum. Not saying you can't do it; just saying that there is not room in this forum for a complete description of all the steps. There are lots of videos and info on-line, search "GE dryer drum removal".