Globe of invulnerability states:
An immobile, faintly shimmering magical sphere surrounds you and excludes all spell effects of 3rd level or lower.
Summon Monster has the following effect:
Effect one summoned creature
Duration 1 round/level (D)
So does the summoned creature count as a "spell effect" and can it thus not enter the area of effect of GOI for the duration of the spell?
Yes, summoned monsters are subject to Globe of Invulnerability, can be dispelled, detect as magical, etc. Do keep in mind that called creatures are not - see the details of the Conjuration sub-schools. If the spell is ongoing, it's simply suppressed while in the globe (the duration continues to go on, and the spell comes back if it moves outside the globe).
Unless, of course, the Summon spell is already in effect when the globe is created, just like every other spell, per the rules of the Globe.
It's not really that confusing, it's just that there are several cases.
Existing - In Place. You create the sphere in an existing spell's area, like a wall of fire on top of you. No effect, the wall of fire is still there.
Existing - You Move. You create the sphere and move it onto some existing spell, like a wall of fire. No effect, the wall of fire is still there.
Existing - They Move. You create the sphere and an existing spell moves into it - a summoned creature or a flaming sphere or whatnot. The spell is not affected and it gets you, as it is a "spell[s] already in effect when the globe is cast".
Newer - In Place. You create the sphere and someone tries to pop a wall of fire or whatnot on top of you. A wall of fire appears with a spherical hole in it. The spell just doesn't function within the sphere.
Newer - You Move. You create the sphere and someone else casts a spell like wall of fire. You move into the wall of fire and the sphere suppresses it within its radius.
Newer - They Move. You create the sphere and then someone casts a spell that moves into it - a summoned creature or a flaming sphere or whatnot. It's suppressed while within the sphere. You can skin that as "can't enter the sphere" or the slightly weirder "it disappears, then the caster moves it back out and it reappears?"
The spell makes no distinction about whether the globe is moved onto a spell or a spell moves into it or whatnot. The net is that any spell preexisting is unaffected and any newer spell can't come into the sphere under any circumstance.
The one end run around this is spells that create a thing that isn't dispellable or whatnot, like some conjuration (calling) spells and instantaneous duration conjuration (creation) spells. So while a wall of fire (evocation) is affected, if a caster creates a wall of iron after you've popped the sphere, it is unaffected by your sphere. This is the standard "conjuration dodge" used to affect creatures with SR, etc. Note that summoned monsters bypass SR simply because the SM spell in particular has "SR:No," but that's a case of specific trumps general. In general, conjuration (summoning) is a magical effect subject to Globe, SR, etc.
In addition to mxyzplk answer, there is some controversy over the meaning of the line "previously cast spells":
http://www.enworld.org/forum/showthread.php?81705-Question-about-the-Globes-of-Invulnerability
Spells of 4th level and higher are not affected by the globe, nor are spells already in effect when the globe is cast.
I'm virtually certain that the authors were thinking of, and meant, "spells already in effect inside the globe's area" (which could include an unwanted Silence, but would also include all of the caster's buffs), rather than "spells already in effect outside the globe's area which are subsequently brought into the globe.
Another person also says:
I don't think that's the way it works. I think the Globe keeps spell effects from entering the area of the Globe regardless of when they were cast. The passage in the text of the spell about not suppressing spells already in effect applies to spells already in effect within the area of the Globe and not ones that might come into the Globe later. By this, I mean that the Globe hedges things out from coming in, but doesn't do anything about spells already in effect with it's boundaries. I suppose it gets a bit tricky about wizards with spells on them who cast the Globe, leave the Globe, and then come back. Does the Globe remember what was and what was not on the wizard? I think it would be easy enough to say yes without assuming the spells have some kind of mutually shared timer.
