At at a high level both Gabi and SimaPro share a common purpose i.e to identify environmental hotspots of product systems at level of assemblies or processes. They mainly differ in terms of the extent of supported databases, methodologies.
Gabi, SimaPro both are detailed LCA softwares that are relatively superior in terms of their computational capabilities compared to stream lined LCA softwares such as Solid works sustainability express or Sustainable Minds.
In the context of energy generation both softwares can be used in principle for various other purposes such as to perform scenario analysis of various energy sources or to perform comparative assessment of energy or material alternatives used in energy generation.
Both are very good software packages. in my opinion, Gabi has a more "process" approach and SimaPro a "product" one. It depends on your preferences. if you can from chemical engineering field, Gabi could be more confortable.
There is also another difference, GaBI has an educational license for free, but SimaPro all licenses have to pay to get it. However, I am using SimaPro!!! :)
I have worked with SimaPro, GaBi, Umberto and openLCA (mostly with Umberto, though I have a personal preference). While openLCA and SimaPro are clearly and only focused on one product, GaBi and Umberto are stronger on process analysis. Both can use common LCI databases like GaBi and ecoinvent. Umberto is particularly strong on non-linear process specifications and system analysis, i.e. assessing the whole system and its interactions before (or even without) breaking it down into single linear product systems. All four approaches that I know are suitable for straightforward LCA, Umberto is particulary useful when you want to explore your production systems thoroughly.