Why Gate, when Summoning will do? Perhaps they don't have access to the appropriate Summon spell. If they dont have access, "monster acts immediately, and attacks your enemies to the best of its ability" is entirely what they would do.
If they do have access to appropriate Summoning magics, there is no reason to utilize Gate, unless the Devil wants more than a few rounds worth of assistance.
Well just for the record, here you go:
PHB 208 - "If you choose to exact a longer or more involved form of service from the called creature, you must offer some fair trade in return for that service. The service exacted must be reasonable with respect to the promsed favor or reward. In general, a gift of 100 gp per HD of of the called creature is reasonable. (Unfortunately, some creature want their payment in "livestock" rather than coin , which may involve complications.)"
Not only do I believe I am technically correct, but from a creating believeable/interesting encounters as well.
Fer the most part, Gating in other creatures won't break Invis.
Sometimes(perhaps rarely) it will.
Occasionally, it may be necessary to 'drop' invis to effectively lead those Gated in.
The only thing I'm saying, I suppose, is that I am not incorrect when I claimed conditional. Frankly I'm suprised that so many people seemed determined to prove me wrong on this point.
Normally, I wouldn't bother to go to these lengths to explain my stance, but JC's surgical strike rules snipe kinda 'Got my Irish up'.
"Daddy, look I drew you a Horsey!"
"One of the eyes is crossed. Here, look at a real picture."
This whole thing may be moot, since according to MM 1, it doesn't seem that Devils have the Gate ability anymore(unless they have sufficient actual wizard levels). They have a simple Summon Baatezu(sp) ability, which works "...much as though casting a Summon Monster spell,..."
No hard feelings to anyone tho, it's turning out interesting.
I must be really bored.