Unfortunately, Gboard doesn't seem to support multiple languages enabled at the same time if they use different scripts - you still have to switch languages.
I can kind of understand why, but I don't think it would be impossible to design a way for it to work.
Can you elaborate on this? I have three languages enabled on Android GBoard and it'll offer up completions and suggestions across all three languages even though I'm usually on English.
I'm talking about languages using different scripts/alphabet, e.g. Latin, Cyrillic, Greek etc. The feature you are describing ("Multilingual typing") is not supported in that case.
So for example, you can enable it for English and Spanish, but not for English and Russian. In fact, it doesn't even seem to be supported for different languages that use Cyrillic.
I can kind of understand why, but I don't think it would be impossible to design a way for it to work.