That gel-like thing is actually the basil seeds - when they come into contact with water, the outside of the seed shell breaks down and is used during the germination process. Some other seeds, such as chia seeds, do the same thing. Some people even eat soaked basil/chia seeds - you may have seen them used in things such as bubble tea.
Hope that answers your question! It will go away once the seedlings appear.
There is no such thing as too many AeroGardens. Nor can you have too many Kratky containers. Avatar: 3x Cacti from Burpee Assorted Cacti (taken Oct 04, 2020)