Can i use my Avent Microwave sterilizer on Evenflo clear plastic BPA-free bottles and Gerber plastic bottles?

Sure you can. Avent and Evenflo are just the brand names for the product. Yes, they show pictures of Avent items being placed in the sterilizer in the manual but it's only because (A) it wouldn't be good marketing to use another brands' products in your literature and (B) they want you to buy all of your products in their brand. Go for it...I'm lending my friend my Avent sterilizer and she has all Medela products.