Sentence 1 is better. However, you should include the word "a". "I have gained a valuable experience"

The noun experience is both a countable noun (which uses an article) as well as an uncountable noun that does not use an article. In both sentences the noun experience is uncountable. Here are a few other uncountable nouns: https://www.englishclub.com/grammar/nouns-un-countable_2.htm Also, "from" can be used once or twice in the same sentence. Using it twice just makes the sentence a little clearer by separating the two different activities (experiences).

Thank you so much. I appreciate your suggestion.

I think these need some work. Maybe this would work: 1. I have gained valuable experience from leading homework correction (and) discussion sections and from instructing (a) SPSS software operation section for undergraduate students. does 'for undergrad students' apply to two parts, or only one? because if it applies to both you can rewrite it: I have gained valuable experience working with undergraduate students leading sections on homework discussion and correction, and on SPSS software operation. it depends also on the context of the sentence, and what flows best