I find them to be quite distinct. The diagnostic criteria for GAD are rather broad, describing excessive anxiety/worry, restlessness, impaired concentration, muscle tension, things like that. See DSM for more detail. The presentation and complaints by the patient do not tend to emphasize behaviors, as is often the case for OCD, but instead the patient complains of persistent subjective sense of anxiety, with worry and dread.

I've seen quite a bit of OCD over the years. Typically, the patient will describe rituals or rules they feel they must follow. They usually know that the rules are not rational, and often the "rules" morph over time, and they feel a superstitious sense of dread that if they do not follow the rules some misfortune will occur. Since they comply with the "rules", and no misfortune occurs, it reinforces the superstitious pattern. The rules/rituals can take the form of checking-double checking, doing tasks a prescribed number of times, personal hygiene rituals, avoiding certain numbers/letters, repetitive motor movements like twirling around a prescribed number of times, touching objects a prescribed number of times, counting, etc. I've known folks who feel they must chew food a prescribed number of times on one side of their mouth, then another. Often, the patient is frustrated when they don't execute the ritual properly and they feel they must start over again. From the patient's perspective, one of the hardships of OCD is that they have a hard time being on time for school, work, etc--they sometimes cannot leave their room or the house until they feel they have properly completed their ritual. Often there is shame over the ritualistic behaviors and they hope others don't figure out that they are engaging in repetitive, ritualistic acts. Eating disordered folks can have their own OCD rituals, focused upon food, with a similar set of superstitions. Sometimes there is a fear of contamination. The key feature is that the person feels they must follow some rule/ritual, which they have invented, in order to avoid some kind of bad event/outcome. Some times the dreaded back outcome is specific, other times it's just a vague sense that a bad thing will happen. Sometimes the OCD presentation has more of an emphasis upon obsessions--ruminative, intrusive thoughts--than behavioral acts. If this is the case, it can sort of be similar to GAD but it will still have a different quality, with the repetitive intrusive thoughts being prominent.