You won't get suds as thick as a 'foam lance' used with a pressure washer:

FOAM GUN WITH GARDEN HOSE


FOAM LANCE WITH A PRESSURE WASHER


But remember though, both has their own pros and cons.
For example, the foam lance would give thicker foam and cling better. Also uses lesser product. But expensive and takes more time to set up.
Foam gun gives out thinner foam, uses more products but less expensive and easier (can just use normal quick coupler).
I can list out a few more differences but, in general, the foam gun is more practical for 'normal' use. FYI, nowadays I seldom use my foam lance anymore.....

Oh...take note though...my house water supply pressure is very high. Even with normal sprayer on jet setting, I can shoot the water all the way onto the roof of my 2-storey house and can easily reach a horizontal distance of 50' ~ 60'. You'd get lesser foam than the picture I posted above if the pressure is lower.