Well, it's of course possible to implement the WebSDK through GTM, but you will likely have to create the whole configuration as well as sending the events to the Platform through custom code. Don't forget that you will also lose the XDM schema visualization in the UI Launch comes with. Meaning, chances to integrate flaws during the implementation are higher as well.
Bottom line: yes, it's possible, but surely not recommended if it can be avoided.
Of course, GTM is much better at integrating Google products, and if the client does not want to use parallel Tag managers e.g., for performance reasons, he might try to avoid a full migration away from GTM.
But yeah, if possible, I would push for Launch over GTM
Cheers from Switzerland!