Linz is a sizeable city and rather difficult to search as there are almost 10 parishes, when assuming your ancestors were Roman-catholics. The duplicates of the records of these parishes are available on LDS microfilms.
The parish records of Mödling and Baden bei Wien have not been filmed. You would need to travel there, correspond with the parishes, or hire a local researcher.
There is an effort underway to digitize, among others, the records of parishes in the diocese of Vienna (which includes Mödling and Baden), and of the diocese Linz. The results are gradually coming on-line. However, it seems to me that Mödling and Baden have not yet come on-line and you would need to wait. You could ask the organizers of this digitization effort about the likely date for these two towns. The web site is called matricula at the address http://www.matricula.findbuch.net/
There might be some problems with the data you have in your posting. If Erich was/is Gisela's son (as you seem to say), the difference in their birthdates cannot be 10 years. Further, how can Erich have the last name Benischek when the father of Gisela, i.e., Viktor, also had the last name Benischek? How certain are you about the spelling Heippel as the last name of your greatgrandmother? The Austrian telephone directory has not a single entry listed for Heippel. Alternatives that do exist are Happel, Hippel, Heupl, Heupel, and likely others.