Not all speedlight gels are equal. The main things to look at are:

• color accuracy and consistency: a proper CTO gel should give a predictable correction, and cheaper generic sets may vary more.
• material quality: better gels are thicker, flexible, and less likely to warp or melt from flash heat.
• size and coverage: make sure the gel fully covers your flash head.
• attachment method: choose something quick and practical for how you work.
• light loss information: some better sets indicate how much exposure is lost with each gel, which is useful, especially with manual flash.

If you mainly want creative color effects and don’t mind experimenting, inexpensive colored plastic sheets can work fine and can be cut to any size. If you need reliable color correction such as CTO, better-made gel sets are usually the safer choice.

So, attachment matters, but color accuracy, heat resistance, coverage, and usable information are what separate better gel sets from basic ones.