"gain" works well with memories, because you will always have those memories. However, it doesn't work well for "scenery," because you only "have" scenery while you are looking at it. SO: We witnessed beautiful scenery and gained many great memories during our trip. Goodbye, beautiful Thailand.

An interesting and (for me) a difficult question! As a native speaker 'gaining scenery' did not sound right but gaining memories felt possible. I had a look on some of the collocation dictionaries (dictionaries of words that go together - for example in Chinese 'big' collocates/goes together with 'rain' but 'heavy' collocates with rain in English). In the dictionaries I couldn't find either word linked with 'gain'! Have a look if you are interested! http://www.ozdic.com/collocation-dictionary/gain I also looked at the word checker I use at my university (sketchengine) and could not find any examples of gain and scenery/memories being used together. So, that's my 'technical' answer. My personal answer would be that I would hear 'gain scenery' as strange and not working for me but 'gain memories' as unusual but making sense to me. Hope that helps rather than confuses!

We acquired, sounds a little bit better than "gained". "To gain" is usually used in a material, physical sense".

It's hard to find a good verb that you can use for both scenery and memories. "Gain" doesn't really work at all for scenery. You might do better to accept that you need two separate verbs, and perhaps choose something like, "We saw (or - "experienced", "enjoyed", "wondered at") beautiful scenery and made (or "created", or possibly even "laid down") great memories during our trip. Goodbye, beautiful Thailand."