I will copy and paste my description for the 100 gap achievement, as you will get the Gap 50 achievement as you progress towards that.
"This achievement is gotten by getting 100 gaps in one run, so make sure you are in Endless Run mode and not the timed 2 minutes one. A gap only counts if it comes up at the bottom registered like it was a trick, there are other times when you will see gap 200 or 300 or whatever pop up, those do not count towards this total.
I got this achievement after 8 minutes on the second level in the game called Rooftops (bought for 150 bucks of the games currency). This achievement could be done on the School level however will take much longer as there are no way near as much gaps. Rooftops has lots more gaps as every rooftop you Ollie off of will most of the time grant you a +1 towards your 100 total.
Now this achievement is quite hard as the Rooftops level is more hazardous compared to School but it really comes down to learning all the sections in the level and knowing when and where to Ollie. It took me an hour and a half of on and off trying to get the achievement as there would always be one little bit I'd mess up on and result in me needing to start over. It's important to notice that the level is always randomly generated with each part having 3 sections (sometimes less). Memorise your preferred lane to be in and keep going on it every time you get to that part of the section.
Finallly It's probably worth making sure your skater is levelled up to the max as some of the gaps require a big Ollie which just isn't possible on a low stat skater. Sometimes you will need to do little Ollies not big ones, that's what messed me up the most as I would clear the gap I wanted to Ollie and then land in to a hazard before I had chance to Ollie again.
Overall the achievement isn't too hard if you're willing to put the time in, just a bit of a pain which I am glad is over for me. Feel free to ask any questions and I'll try my best to answer."