This is how I have my gallery set up right now:
If(CollateralTypeDrop.Selected.Value="All",
Filter(ContentIndex,ContentDesSearch.Text in 'Content Description'),
Filter(ContentIndex,And(ContentDesSearch.Text in 'Content Description', CollateralTypeDrop.Selected.Value in 'Collateral type'.Value)))

(ignore the dropdown input on the right for now, its not live yet)
So far I can filter nicely using one word in the text input (ContentDesSearch.Text), and combine that with a filter in the dropdown (CollateralTypeDrop)
Need to be able to use multiple words in the text input though. Haven't been able to figure this out using other answers found here.
Any tailored help would be massively appreciated!