You can use the If function to either have all events (if the choice is "all events"), or filter based on the event type. For example, those would be the properties you'd need to set:
Dropdown1.Items: ["Internal", "External", "All Events"]
Gallery1.Items:
If(
Dropdown1.Selected.Value = "All Events",
Events,
Filter(Events, EventType.Value = Dropdown1.Selected.Value))
That should work for SP lists.
Another option is to have the "All Events" have a value that will match any items - for example, using the StartsWith function. In the example below, if the user chooses the "All Events" option in the dropdown, the gallery will have all items whose event type starts with an empty string (and all valid strings start with an empty string), effectively returning all events.
Dropdown1.Items:
Table(
{Value:"Internal", ColumnStart: "Internal"},
{Value:"External", ColumnStart: "External"},
{Value:"All Events", ColumnStart: ""})
Gallery1.Items:
Filter(
Events,
StartsWith(EventType.Value, Dropdown1.Selected.ColumnStart))
