I wonder if we can use 'besides+to+verb' or if it is only 'besides+verb+ing'
Besides to get such data, we must also ...
besides getting such data, we must ....
Which one is correct ? What is the most formal sentences among the two ? Is 'besides' adapted here ?
True as said by Josh. The preposition besides in such structure takes gerund after it and not an infinitive verb.
besides - in addition to someone or something; except for someone or something.
Check the examples given by OALD, it takes the gerund after it.
Also note that, besides can also go this way in your case...
We must also [whatever your concern is] besides (such) data.
Other ways of using besides are here.
