To answer the question whether Gelug practitioners are sectarian, I would like to go back to the definition of what is sectarianism. Sectarianism, according to one definition, is bigotry, discrimination or hatred arising from attaching importance to perceived differences between subdivisions within a group, such as between different denominations of a religion, class, regional or factions of a political movement.
One choose a particular lineage because it suits one's disposition. In Tibetan Buddhism, one is encouraged to investigate prior to committing to any teachings and practice. If this pre-requisite is executed properly, then it can safely be said that one choose to enter the lineage because it suits one's disposition. There is nothing within the gelug teachings that encourage one not to respect and to hate other true religious denomination.
In that sense I don't think we are secterian
Gelugpa was never sectarian from the start, so how can it be sectarian now? The sectarianism that is perceived of Gelugpas comes from nothing but pure ignorance and jealousy as the Gelugs are after all the fastest growing tradition compared to the other traditions. It is also the tradition with a very complete set of teachings from the rest of the traditions and a very solid and stable system in place to keep the teachings intact.
Now, why would people perceive exclusivity as sectarian? Mainly due to ignorance. Ignorance of how things work and also what sectarianism really is. There is also the popular belief that is prevalent these days that being non sectarian is to mix everything up together, and it is not sectarian because it shows that you are accepting of other people's tradition and teachings. Or rather, that is what people would like to believe.
So why is that belief a bad one? Because it causes people to not be stable and encourages people to mix things around. If you would not mash up instructions on how to bake a cake with the instructions on how to bake a pie, why would you want to do that with the Buddhist teachings? Every lineage itself has a system within that should not be touched or messed up with. Gelugpa is one of the lineages that are sensitive to being messed up as Tsongkhapa's system is already perfect.
Other traditions may not be exactly be too sensitive in this regard, but what applies to them may not apply to the Gelugs. There is definitely a very good reason why Jamgon Kongtrul did not include Gelug within the Rime movement: because Gelug is Rime by itself.