When you say 'tips' do you mean over acrylic or hard gel extensions that the tips have been used with or applying tips then using Gelish alone over it?
If you want to extend the nail with a tip, you can use Gelish structure gel over the tip to add strength, but not too long say 3 or 4 mm at most. Use Gelish structure Gel after foundation, so prep in the normal way,apply tip blend as normal, cleanse (avoiding using clense on the plastic tip as it may cause it to crack) apply PH Bond t nat nail area only, apply foundation gel all over & dry wipe, then use Gelish Structure Gel to add structure to the extended nail/tip. Then go with your colours.
Gelish on it's own is not strong enough too be used with tip extentions unless using the Gelish Structure Gel.
You can however, use Gelish over acrylic or hard gel extensions, just finish the enhancement to a 180 grit finish, wipe with Cleanse and apply colours.