Could it have turned to gel because a few days went by? Do you normally use it right away? If you normally use right away and it's liquid, but this time you waited a few days and it was gel, i think it makes sense that the consistency just changed over time. If it's a product that has always worked well for you and given you good results, then I think it's fine to use.
Depending on the zone you're in, you may always have insects and fungus. In Louisiana, we pretty much do. This might not be the case in less humid zones.