As I understand it, a "gish" has two main components that must be achieved by level 20: BAB +16 and level 9 spells. There are a couple of exceptions (like Swiftblades), but that's the general rule. As much multiclassing as there is between melee and caster classes, saves tend to take care of themselves.
Part of the point of a gish is to sacrifice one form of versatility for another. You give up some of your spells known/spell slots for the ability to pick up a weapon and wade into combat when spells aren't as effective.
A gish wouldn't fit in a T5 campaign, as the closest thing to a T5 spellcaster is an unoptimized Truenamer. I probably wouldn't try one in a T4 either since that's where the Adept and Paladin like to hang out, and a gish build would step on both their toes.
For your example of level 6 spells and maneuvers, that would probably fit in around T3 with the Bard, Psychic Warrior, and Duskblade, although I generally wouldn't consider it a gish. This would probably also be a good tier for characters that would fulfill the technical requirements of a gish that I outlined above but get a slow start. One example would be a build I tossed together but never played: Bard 4/Crusader 1/Spellsword 1/Jade Phoenix Mage 2/Abjurant Champion 3/Sublime Chord 2/Jade Phoenix Mage +8. You get one level 9 spell at level 20, level 5 spells don't show up until level 11, Sublime Chords get fewer spells per day than Sorcerers, you get very few maneuvers (very late, to maximize Initiator Level) from a limited selection of disciplines, Abjurant Champion doesn't synergize very well with Bard, and you'll probably need Able Learner and a decent Intelligence to hit the Sublime Chord prerequisites with such limited skill lists and dearth of base skill points.
True Gish builds (those conforming to the requirements above) are less overpowering at T1 and T2 and can probably be classified according to how they get their spells. A good T2 gish might be Paladin 2/Sorcerer 4/Spellsword 1/Abjurant Champion 5/Sacred Exorcist 8. You still put off level 9 spells until level 20, but you have a lot more synergy-- Charisma to all saves and number of turning attempts (for Divine feats), Spellsword's ASF reduction actually accomplishes something for Sorcerer, and Sorcerer works better with Abjurant Champion. However, due to the nature of Sorcerer casting, you're limited in the number of tricks you can pull off, and a good number of your spells will probably involve buffing yourself rather than playing battlefield control. A Wizard-based build may fit better into a T1 game thanks to the flexibility of changing your prepared spells every day and lack of hard limit on unique spells known.
The archetypal psionic gish at this point would probably be Psion 6/Anarchic Initiate 4/Illithid Slayer 10-- notable for only losing one manifester level. It's also pretty light on required feats and skill points and uses a better casting stat than the Sorcerer build above. Depending on your power setup, this one could fit pretty well into a T1 campaign. (You'll probably be relegated to the Big Dumb Fighter role, but you'll be one of the smartest BDFs around.) Depending on your power setup, you can abuse the action economy almost as much as a Wizard, Ruby Knight Vindicator, or Swiftblade. (Specifically, he can use Expanded Knowledge: Dimension Hop (or Hidden Talent if it's allowed) + Divert Teleport for cheap, action-efficient tactical-level teleportation and Schism + Control Body + Solicit Psicrystal to essentially take two and a half full rounds of actions per round-- these are some of the archetypal tricks of the Chrono Legionnaire. The less said about Metapower Synchronicity Linked Power Synchronicity, the better-- it's basically an infinite action loop until you run out of PP. Recharge setups may allow him to blow nearly all his PP each encounter, if he can control how much time he has between encounters.)
Speaking of the Swiftblade, I called it out at the top as being an exception to the BAB +16/level 9 spells rule. You can build a Swiftblade that gets level 9 spells but it's one of the few classes that's worth giving them up because the capstone lets you cast Haste out of a level 6 spell slot for a one-round time stop, with higher-level spell slots increasing the duration by one round each. A level 8 spell slot would stop time for 3 rounds, roughly the expected duration of a regular Time Stop. The archetypal Swiftblade build would be Wizard 6/Swiftblade 10/Abjurant Champion 4, which gets BAB +17 and caster level 16. (Sorcerer also works. If you wanted, you could also start with Transmuter 4/Master Specialist 2, although you'd probably want to start with a race that grants some martial weapon proficiencies.)
Another thing to consider for Gish builds is what level they're going to see play. For the first six levels, the Swiftblade is behind the power curve due to what look like poor feat choices, and the Paladin/Sorcerer is probably about on par with a single-class character because of the efficiency of low-level physical combat and the effectiveness of low-level buffs. At the transition from low to mid levels, gishes will still be a bit weaker than full casters because of the resources "wasted" on developing their combat abilities. Even at high level play, they may still technically be weaker than full casters, particularly those that take full casting prestige classes to inflate their power further (or even learn spells to duplicate gish abilities), although a gish may require less maintenance to keep him fighting effectively.
You may notice that I haven't mentioned any divine casters. That's because Cleric and Druid are basically gish-in-a-can. Cleric can cast Divine Power and have full BAB (alongside full casting), while Druid can Wild Shape and, since a full attack now consists entirely of natural weapons, basically ignore BAB except for Power Attack limits (and also have full casting).
Hopefullly, someone else will come in behind me and fill in some of the gaps. So far, I haven't had much experience at playing a high-level caster, and the highest-level caster I have played certainly wasn't T1. (Maybe a low T2.)