There isn't really a verb form. You say things like 'I am the goalkeeper/goalie for X...' or 'I play in goal for X.' 'To keep goal' is another phrase that is sometimes used. I keep goal for... But that sounds kind of formal, more like the kind of thing that would be used in writing than in speech. 'Guard' sounds extremely strange to me, I guess Americans use it because of parallels to sports like basketball and (American) football where that is the verb of choice. In a UK context that verb would not be used.

The verb form is not often used. If you had to say it it would be "I was goalkeeping or He is goalkeeping for his team." But usually other words are used. "Guarding" works well for this. Or you can just say "He is playing as a goalkeeper. This is what Americans would say. Its possible that the British have a different method because soccer is not played much in the USA.

I am not sure I would 100% agree with tjr other answers here. The verb is to 'goalkeep'. This is not often used but we shorten it to 'keeping' quiter a bit.