Unlike 1000 Thumbs achievement, the 1000 thumbs for Mega Upload have to obtained on a single upload. Getting them may be a challenging task if you don't make a popular upload. Some people had luck with uploading popular high resolution wallpapers. You can also promote your achievement in certain threads, one of which I created in TSA is available under
Garry's Mod Thumbs achievements .
You can check your current progress on your upload without launching the game by clicking the "Item Stats" button on your workshop and checking the "POSITIVE RATINGS" number.
Progress toward the achievement updates shortly after starting the game and can be seen in notification in the upper-right corner, but can also be checked in console by launching the game with
+achievement_debug 1 launch option and will update periodically shortly after each thumbs is received. The game also keeps number of thumbs earned in "MOST_WORKSHOP_THUMBS" value, which can be checked via Steam API request
http://api.steampowered.com/ISteamUserStats/GetUserStatsForG...where "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" is a Steam API key (get yours at
https://steamcommunity.com/dev/apikey page) and "YYYYYYYYYYYYYYYYY" is your steamID64 (see
https://steamid.io/ or similar).