Travel was very difficult and as such, Kings rarely met each other in person let alone twenty of them meeting in a summit. Even beyond the medieval era, early modern kings still seldom met each other.

Charles V met Francis I of France because the latter was captured and brought to Spain. They met a second time when Charles needed to get to the Low Countries in a hurry, and Francis invited him to visit Paris and travel through there instead of going by sea.

Francis I met Henry VIII only once, during the meeting at the Field of Golden Cloth, to set up an alliance against Charles V. The two young dashing kings danced together and spent the night together as a sign of trust.

Charles V met Henry VIII maaybe once, either when his sister Catherine of Aragon was first married to Arthur the elder brother of Henry VIII who then deceased, or when she was then married off to Henry VIII the next in line for the English crown.

And Charles V was considered one of the most well traveled kings of the early modern era.

In the medieval era, kings sometimes saw each other while campaigning on a Crusade. But I wouldn't call those a G20 summit.

the Closest thing would be the a church summit or election of a HRE. where the highest highs of german dukes would gather.

In a way each county's House of Lords/council of state was a G20 summit. the world was smaller - but each of those great lords was a world economy unto himself. In some countries the Lords might own up to 50% or more of the land/wealth of the country.

So the Duke of Norfolk might easily be equivalent to the GDP of Belgium in the smaller world of medieval states (and given the independence of medieval lords - just as independent in policy).

Medieval lords also had their own armies, trade relations, tax systems. and in places like france and germany - may actually be far wealthier than the King.