Seachem seagel is a blend of their martix carbon which is very good and gfo. it should work fine but the gfo (phosguard) and carbon will exhaust at differnt times so while you can run them in the same reactor ,filter or bag as a convenience, it's easier and more efficient to run them separately.
Chemi-pure comes in two flavors:
Chemi- pure contains an average granulated activated carbon and some deionization resin. The resin is not helpful in slatwater;it doesn't hurt anything in a significant way but with all the dissolved solids in salt water the resin eshausts very very quickly and grabs some good stuff as it does. The resin is helpful in freshwater applications.
Chemi-pure elite contains the resin and the gac like regular chemi- pure with some gfo added.
Personally, I prefer rox 8 carbon and HC Gfo from Bulk Reef Supply.