The Marshall Gambit is a dream for Black if they like this type of counter-attack and the resulting positions. Once you are familiar with the basic plans for Black, the Marshall Attack is quite easy. You don't have to memorise everything in the form of variations. A few well-studied patterns, the ability to calculate well and you will be able to solve many things at the board, at least below professional chess. Unfortunately, you will also have to reckon with the fact that the Marshall Gambit will not always be possible.
With White, you should definitely decide before the game whether you will boldly accept the gambit or prefer to play one of the popular anti-Marshall systems. These often lead to position types that are familiar from the closed Spanish and there are also frequent move swaps. From a pragmatic point of view, this is usually the better solution for White.
Would you like to learn this opening? You can find specialised opening training courses here: Marshall (chessbase.com)