Learn Swedish: Gänget brukade samlas på skolgården varje morgon. - The gang used to gather in the schoolyard every morning.

Samlas actually is the bare infinitive! It is an s-verb, which in this case means it has a reciprocal meaning (people gathering together). The rule for brukade (used to) is that it must be followed by a bare infinitive, and for this verb, the infinitive naturally ends in -s.

Why do we use på for 'in the schoolyard' instead of i?

In Swedish, you generally use the preposition på for open surfaces, areas, and yards. Because en skolgård is an outdoor, open space, you use på skolgården. You would use i if you were enclosed inside a building, like i skolan (in the school).

Why doesn't morgon have a definite ending after varje?

The word varje (every) always takes the indefinite singular form of the noun that follows it. So even though you are talking about specific mornings in a routine, it must be varje morgon, not varje morgonen.

I know gänget means 'the gang', but what is the base form of the word?

The base word is ett gäng, which is a neuter noun. Because it ends in a consonant, you add -et to make it definite (gänget).