Google Gears and Linux
“Google Gears” could not be installed because it is not compatible with
your Firefox build type (Linux_x86-gcc3). Please contact the author of this
item about the problem.
Found the fix in this Google group post which says re-installing Gears should fix this issue.
On more digging, it appears that there are several bugs reported with this issue e.g. Bug # 975, 977. Apparently someone uploaded a custom gears xpi to fix versioning issue. Google indeed has moved on to phase out Gears in favour of HTML5.