Gate has always seemed a bit too good to me, and the removal of XP costs (while an excellent decision overall) has rendered it more than a little broken in my opinion. I'm talking about the function to summon a monster to perform an immediate service, as I don't have any experience with long-term services.
So, to start. As an example, with a ninth-level spell slot, you can either:
1. Cast Meteor Swarm
or
2. Cast Gate to summon a Pit Fiend, who will subsequently cast a Meteor Swarm himself, and then proceed to be at least as useful as another character for the rest of the combat.
And that's just the intended use of the power, which I'd already argue is just too much. Moving onto only slightly debatable uses, you can:
-Summon any Great Wyrm short of a Gold (provided the caster is not on the material plane)
-Summon an advanced Pit Fiend/Balor/Solar/Whatever with hit die up to the maximum allowed by your caster level
These monsters are far more powerful than any individual character, and quite possibly more powerful than the entire party. When you run into a CR 22 encounter, you can throw a CR 26 at it as a standard action with no gold or XP cost.
The result of this is that encounters after the party hits level 17 just degenerate into Gate wars. Often the solution is found in stupid and arbitrary restrictions on all forms of summoning in various locations, which doesn't leave anyone happy.
With regards to balancing Gate, my thought is that keeping a summoned monster bound to whatever plane you're on and under control would require concentration. The monster remains present and at your command so long as you concentrate on it; once concentration is broken it regains free will and remains present for d4 rounds (and is probably rather pissed off at you). The intensity of concentration required might be based on hit die and reflected in whether you must concentrate as a standard (up to caster level in HD) or full-round (up to double caster level in HD) action. The caster's saves and AC might drop while the spell is in effect, to an extent also dependent on the HD of the summoned creature.
This doesn't leave a provision for long-term service as that's seldom used anyways and I believe it's what Planar Binding is for. What I'm going for here is a spell that allows the summoning and control of monsters about as powerful as you want in battle, but at some risk to yourself and not as a free 40HD advanced Solar cohort who shows up to help you with every fight. Thoughts?