Since you're a self-professed grappling n00b, I'll try to explain this with as little technical detail as possible. Both can work, but require some combination of passivity or poor defense by the victim, and high levels of skill/control by the attacker, as well as flexibility in the case of the gogoplata.
With the Gogoplata you'll frequently see someone just yank their trapped arm out, or turn a bit to relieve the pressure. Also, good defensive positioning within the guard can negate the attempt itself.
The omoplata, on the other hand, does get used in MMA (not enough imo), but both in MMA and pure grappling it's more often a sweep. In order to finish someone, you have to get them flattented out, or else they can roll to escape as a last resort, and sometimes that can be done even if they are flattened. If they recognize it in time, there are a number of escapes, and it takes someone who is really skilled at it to get a submission with the omoplata.
The good thing about the omoplata is that if the person applying it is thinking ahead, it's a very good transitional move. First of all there are transitions to things like the armbar, triangle, and toe hold, which are all much higher percentage submission finishes. Second of all, many of the escapes (assuming the attacker is on his game) result in a sacrifice of position. So if you roll to escape you just got swept, and that's good enough. So you force them to defend by letting you get top position. Of course you'd rather give up position than get submitted, so that's what often happens. As a result, an omoplata is frequently a sweep rather than a sub at the high levels, and most guys in the UFC are generally pretty good grapplers.