Is there a disadvantage to using gel-type/form of Minoxidil instead of its foam and liquid counterpart?

first of all your age is  very important . Receding hairline is not a hair loss because of ongoing process of androgenetic alopecia but  a simple juvenile type maturing of frontal hair line.  That is seen almost every young boys almost between 16-21 that have a very rounded hairline without recession in the corners. You may try to use any kind of minox. Form is not important but  dosage and proper application is important 

Yes, minoxidil can be used in the temples in males with androgenetic alopecia. It is less effective than use in the crown. Studies that lead to its approval were originally focused on the crown and that is why the focus is the crown. Studies have shown it can benefit the temples (see references below) so this is really nothing new. Many young men confuse balding with hairline maturation so be sure you’re clear on which you have. Minoxidil does not help hairline maturation. There are many formulations of minoxidil and you can choose which one suits you best.
STUDIES REFERENCED
Hillman K et al. A Single-Centre, Randomized, Double-Blind, Placebo-Controlled Clinical Trial to Investigate the Efficacy and Safety of Minoxidil Topical Foam in Frontotemporal and Vertex Androgenetic Alopecia in Men. Skin Pharmacol Physiol. 2015;28:236-244.  
Mirmirani et al. Similar Response Patterns to 5%Topical Minoxidil Foam in Frontal and Vertex Scalp of Men with Androgenetic Alopecia: A Microarray Analysis. Br J Dermatol. 2014 Sep 10. 

I believe that minoxidil liquid is better to use then the foam because more of it can be applied to the scalp where it is effective rather than on the hair where it is wasted