In Sanskrit grammar, each सुबन्त (grossly interpreted as a noun) has 7 (+1) cases and its form will vary as per the case. This is unlike Hindi (which uses something similar to the prepositions in English) but many south-indian languages will find similarities in the different forms and modifications of the words.
So the word सवितृ is the root of a ऋ-कारन्त पुल्लिङ्ग सुबन्त. The word सविता is the first case singular.
It's declension is somewhat like this:
Singular Dual Plural
1. सविता सवितरौ सवितरः
1a. (हे सवितः हे सवितरौ हे सवितरः)
2. सवितरम् सवितरौ सवितॄन्
3. सवित्रा सवितृभ्याम् सवितृभ्यः
4. सवित्रे सवितृभ्याम् सवितृभ्यः
5. सवितुः सवितृभ्याम् सवितृभ्यः
6. सवितुः सवित्रोः सवित्रूणाम्
7. सवितरि सवित्रोः सवित्रुषु
So, depending on what the intent is, different forms will be used. The सवितुः used in the Gayatri will mean 'from the Sun' or 'belonging to the Sun' depending on whether it's the 5th or 6th विभक्ति. And in the case of the Sun, there are no two or more Suns, so the dual and plural cases are not used (unless some respect is to be shown, in which case the plural may be used).
For the पितृ, the declension is similar since it is also a ऋ-कारन्त पुल्लिङ्ग सुबन्त. Context matters behind the usage of this word. The word पिता (1st विभक्ति singular) means Father, while the (1st विभक्ति) plural form पितरः (meaning Fathers) is usually used to indicate ancestors. Note that the (1st विभक्ति) dual form पितरौ (meaning two Fathers) is usually used to indicate मातापितरौ (father and mother).
