its all arbitrary
there is no definition but generally people put tier 1 as top 10 teams / the teams in the highest league / teams that make challengers playoffs and teams that actually got a real chance at winning
tier 2 generally is salaried teams (can be an f/a team if they are great) and they are outside of the top 10 / highest league
tier 3 is generally semi-pro unsalaried players or free agent teams
it can also be separated into world wide tiers and regional tiers also, tier 1 world is different to tier 1 brazil for example
a gatekeeper team is one that is very consistent at beating tier 2 teams but also very consistently loses to tier 1 teams