I have kept tropical and marine for 30 years but never a Gar so I don't make out I know about them which is why I ask for advice on here but I do know how other fish react in certain circumstances.
You could put a severum in and there would be no problem but on the other hand it could be a disaster.
There are obvious scenarios that are or would be a disaster in mixing certain tank mates but its not obvious as it really could be just down to personality.
EG.
I keep a Fahaka Puffer, he would and has destroyed anything in his tank, even a 2ft armoured pleco would be sliced in half. I like him though all the same.
A good friend of mine has a Fahaka too which shares its tank with an Oscar.
I tried that and got the Oscar out very quick.
In 30 years I have learned that fish have different characters so it's never that simple
but opinion is always taken on board....Thank you for your thoughts